IBM has brought Lotus Notes to the Linux desktop a full year ahead of schedule - it wasn't expected until the next major release of a new Notes version - in 2007.

Still, even if a year early, what took IBM so long? The company said that about 5,000 of its employees had been using Notes on Linux as part of the IBM's internal beta test program. And it's no secret that IBM is planning a steady transition to the free OS for many of its corporate workstations.

IBM ported Notes to Linux by using the open source Eclipse framework - which IBM created - to build the client GUI. Underneath the Eclipse widgets, however, there's a lot more going on.

Notes is much maligned as an e-mail client, and its market share is declining versus more traditional messaging systems, such as Microsoft Exchange. But Notes, when combined with the Domino server upon which it relies, is about a lot more than e-mail.

It's a platform for building complex, database-driven collaborative apps. Notes makes it easy for mid-sized businesses, in particular, to build custom back-office applications on a tight budget.

The Linux client for Notes isn't a freebie, nor is it open source. But that's okay. Nowhere in the Linux playbook does it say that commercial applications shouldn't play nice with open source.

Just being able to run Notes on Linux at all is a coup, as it gives Notes-centric workgroups hope for an alternative to the Microsoft licensing structure for the first time. Tellingly, the Notes client licences are fully transferable; if you're running Notes on Windows today, you can re-install Linux on the same machine tomorrow and continue using the Linux version of Notes without paying any extra fees.

Of course, IBM won't tell you that it wants to encourage its customers to switch from Windows to Linux. Rather, let's just say that IBM doesn't think it's a bad idea.

But if you're a Notes customer who has been considering alternatives to Windows, this is big news. If you're not a Notes customer, you might consider becoming one. The addition of Lotus Notes brings a full, rich enterprise collaboration platform to Linux.

It joins the broad catalogue of mature, commercial-grade applications already available on the free OS, such as the OpenOffice.org productivity suite, the Firefox browser, and the Gimp image-manipulation program.

Even better, arguably for the first time, customers can choose from a number of first-rate Linux desktop environments to run these applications on. I'm a big fan of Ubuntu, the Debian-based Linux distribution founded by South African Internet billionaire Mark Shuttleworth.

But Novell's forthcoming SuSe Linux Enterprise Desktop (SLED) is something else again. It's smooth and polished, the result of extensive user testing and considerable in-house development by Novell.

No mere hobbyist's toy, SLED feels like a bona fide commercial OS, addressing a criticism that's often levelled against Linux distributions.

With the release of products such as SLED and Notes for Linux, it's plain that vendors such as IBM and Novell aren't playing around anymore when it comes to providing alternatives to Windows on corporate desktops.

With uncertainty around Windows Vista growing for many enterprise customers, the market opportunity is as plain as day. This is not a drill. Shots have been fired.

So, dare I say it? Do I utter those five fateful words? "The year of desktop Linux" has been declared so prematurely, so often, that it's become something of an industry in-joke. But one thing I can guarantee you: This year, Microsoft won't be laughing. Not even a chuckle.