There's a scene in the 2004 movie Mean Girls in which the most popular girl in the film's fictional high school finds out that a friend who now is a fast-rising social rival plans to throw a party without inviting her.

"Who does she think she is?" sniffs the suddenly-threatened clique leader - or "Queen Bee," in the movie's parlance. "I, like, 'invented' her, you know what I mean?"

Take away the American slang and substitute "open-source" for "she," and you have an approximation of Microsoft CEO Steve Ballmer's declaration almost exactly one year ago that Linux (playing the role of the movie's "Wannabe") "uses our intellectual property."

Ballmer's statement - along with follow-up claims by Microsoft executives that they had found violations of 235 patents in Linux and other open-source software - caused a sudden refrosting of what had been a slowly thawing relationship between the company and the open-source community.

Over the past several years, Microsoft - whose combative CEO once called Linux a "cancer" from an intellectual property standpoint - has set up its own open-source testing lab, begun hosting open-source projects for free on its CodePlex website and signed partnership deals with open-source vendors such as Red Hat's JBoss division, MySQL AB, SugarCRM, XenSource and Zend Technologies.

But by dangling the threat of patent-infringement lawsuits over the heads of users and vendors alike, "Microsoft opened up a can of worms with the open-source community that they have been attempting to close since then," said Charles King, an analyst at Pund-IT.

So the two sides remain wary "frenemies." And their friend-or-foe relationship has continued to evolve in both directions this month.

Microsoft did finally get an invitation of sorts to the open-source party on 10 October, when the Open Source Initiative (OSI) approved two of its software licences as valid means of distributing open-source technologies.

The company also continues to try to ingratiate itself with open-source backers. At the Web 2.0 Summit several days after the approval of the licences by the OSI, Ballmer promised an audience composed primarily of executives from high-tech start-ups that Microsoft would "do some buying of companies that are built around open-source products." That is something the software vendor has never done before, despite its long track record of acquiring technologies and swallowing start-ups.

Early in the week, Microsoft agreed to give developers of open-source workgroup server products access to Windows interoperability information and to slash the royalties that it will charge for using the information from 5.95 percent of a product's revenue to 0.4 percent. But that agreement was a grudging one, made to finally comply with a 2004 anti-trust ruling by the European Commission.

Microsoft also announced a collaboration and "intellectual property assurance" deal with TurboLinux on Monday - the latest in a series of controversial licensing agreements that has split the Linux camp between vendors that have agreed to the terms and others that have said they aren't interested in doing so.

Nobody believes that Microsoft has suddenly displaced IBM as the BFF - best friend forever - of open-source vendors.

Dave Gynn, director of enterprise tools and frameworks at open-source consulting firm Optaros, said he thinks Microsoft is carefully choosing open-source partnerships that it believes will limit the methodology's overall spread.

"Microsoft only does one partnership in each space to ensure there is always one open-source option, though they try not to make it more attractive than their own offering," Gynn said. If Microsoft really wanted to promote interoperability, he added, the company "wouldn't go and announce a partnership. It would just go and do it."

King also remains sceptical about Microsoft's intentions. The company "is endeavouring to be friends with customers that are purchasing heterogeneous computing solutions and using Linux," he said. "However, I'm not sure that makes them a friend of Linux." And, he noted, Microsoft's patent claims are still an issue.

But Adam Solesby, chief technology officer and co-founder of StudioNow, a start-up that's developing a Web-based video editing service for consumers and small businesses, said he isn't worried that Microsoft will go after companies like his.

The spectre of the software vendor suing users "is not a realistic threat," he said. "I think we're fairly safe from that." Solesby is a long-time Microsoft customer whose new company uses a MySQL database running on top of a home-grown version of Linux. But, he noted, he chose those technologies for cost and productivity reasons - not because he has "converted" to open-source philosophically.

Ballmer's description of Linux as a cancer was made in a 2001 interview with the Chicago Sun-Times - although he also described the open-source operating system as "good competition" that would force Microsoft to innovate and justify what it charges for its software.

In contrast to the cancer remark, Ballmer's new comments about buying open-source vendors "set the right tone," blogged Matt Asay, an OSI board member and vice president of business development at Alfresco Software, a developer of open-source content management software.

Now, Asay added in his posting, the software maker "just needs to behave in such a way that open-source companies won't blanch at the thought of being acquired by Microsoft."

In a blog posting that announced the OSI's approval of Microsoft's so-called shared-source licences, Michael Tiemann, the open-source group's president, said that the software vendor had submitted the licences under the same policies and procedures that other parties have used.

"Microsoft didn't ask for special treatment, and didn't receive any," wrote Tiemann, who also is vice president of open-source affairs at Red Hat. "In spite of recent negative interactions between Microsoft and the open-source community, the spirit of the dialogue was constructive."

The OSI received nearly 400 emails when Microsoft first announced in late July that it planned to seek open-source certification of its licenses. But since the OSI approved the licences, it has received "surprisingly few" messages, Tiemann said this week.

In keeping with the nature of the relationship between Microsoft and the open-source community, the emails sent thus far have all been against the decision, Tiemann added.

That "was not a surprise," he wrote. "Most of the folks who agreed with our decision to approve the licences were not huge fans of Microsoft, and thus there was little for them to celebrate when their expectations of a fair process yielded a fair result."

In a statement about the OSI's decision, Bill Hilf, general manager of Windows Server marketing and platform strategy at the software vendor, said the licence approvals were "a significant milestone in the progression of Microsoft's open-source strategy and the company's ongoing commitment to participation in the open-source community." Microsoft said Hilf was on vacation this week and unavailable for a follow-up interview.

"Microsoft appears to have accepted that Linux - on servers and devices at least, if not the desktop - cannot be completely stopped," said Daniel Egger, CEO of consulting firm Open Source Risk Management.

The software vendor also seems to have recognised that enterprise users are demanding "some form of co-existence between Microsoft products and open-source," Egger added.

Novell was the first Linux vendor to sign a collaboration and licensing deal with Microsoft last November, just two weeks before Ballmer made his intellectual property claims. The agreement was criticised by many open-source advocates. But it has borne fruit for Novell, which said last month that its Linux sales grew nearly 250 percent year-to-year in the first three quarters of the fiscal year that ends on 31 October.

Overall, Microsoft has reached agreements that include promises not to sue the customers of seven vendors, including Novell, TurboLinux, and fellow Linux distributors Linspire and Xandros, plus three makers of devices that include embedded versions of Linux.

But those patent-protection deals could help to bolster Microsoft's infringement claims - potentially posing legal risks for users of Red Hat and other Linux vendors that haven't signed agreements with the company.

The deals that Microsoft has inked are "a way to grab some control of Linux," Gynn contended. "The longer [that Microsoft's management team] can continue to delay Linux's adoption or thwart it, the longer it will be good for their Windows business."

But Egger noted that while Microsoft may be rattling its sabre at Linux vendors and users, it has yet to draw any legal weapons. Instead, the first Linux-related patent-infringement lawsuit was filed earlier this month against Red Hat and Novell by a pair of small firms that specialise in acquiring, licensing and enforcing patents.

The emergence of such lawsuits "is one inevitable result of open-source becoming a major economic force in IT," Egger said. "It is not caused by Microsoft, and it will continue long after the current battle lines between Microsoft and Linux are [made obsolete] by technological changes."