Palm, Microsoft, and Verizon will launch a long-rumoured Windows-based Treo smartphone at a press conference in San Francisco today.

The Windows-based Treo 700w has been discussed and even shown on enthusiast websites, but the companies have still not released any official details.

The announcement, which marks a break with the history of Palm's devices, will be made by the three companies' top executives: Palm chief executive officer Ed Colligan will be joined by Microsoft chairman Bill Gates and Denny Strigl, Verizon's chief executive officer, at the Palace Hotel in San Francisco.

Ever since Palm separated from PalmSource, the company that develops the Palm OS, in 2003, analysts and enthusiasts have been speculating that Palm would eventually turn to a different operating system vendor for its smart phones and personal digital assistants. That speculation reached fever pitch earlier this week, when Engadget (and even shown) posted photos of the unannounced Treo 700w running the Windows Mobile operating system.

Palm, which virtually invented the handheld computing market, has been linked to the Palm OS since its inception. But Microsoft has made steady progress with its mobile operating system, and handheld vendors actually shipped more Windows Mobile-powered devices during the last quarter than Palm OS-based devices, according to Gartner.

Microsoft's ability to link Windows Mobile to its enterprise software, such as Exchange, makes perfect sense for Palm, which has been trying to break into the corporate market for a long time, said Sam Bhavnani, senior analyst with Current Analysis in San Diego.

The Treo, a PDA which can also make voice calls, is one of the hottest selling mobile devices on the market, Bhavnani said. But with the support of Microsoft, IT managers might find it easier to issue the devices to their mobile workers, since they can take advantage of their familiarity with Microsoft's products, he said.

The Palm OS had been facing an uncertain future, with the ">purchase of PalmSource by Access, a Japanese mobile Web browser company. PalmSource had said it was planning to turn to Linux as the base for future versions of the operating system (read our interview with David Nagel, the PalmSource CEO who quit in May). .

Palm and PalmSource had hoped to use Cobalt, the code name for a smartphone version of Palm OS, in an upcoming Treo, but it never came to pass, said Todd Kort, principal analyst with Gartner. Palm chose to use Garnet, an older version of the Palm OS, in the Treo 650, and was forced to do a significant amount of software development on its own to make Garnet suitable for a phone, he said.

The Treos have enjoyed a good run in recent months, but Motorola signalled its intentions to challenge Palm with the Motorola Q, a Treo-like device that will run Windows Mobile 5.0 when it is released early next year.

"If Palm didn't make a deal with Microsoft, it would face a challenge from that device," said Brad Akyuz, an analyst with Current Analysis.

The other major operating system choice on the mobile smart phone landscape is made by Symbian, which is partly owned by phone giant Nokia. The Symbian OS is the world's leading smart phone OS, and is very popular in Europe and Japan, where mobile phone data networks are much more advanced than in the U.S.

A Palm spokeswoman did not immediately return a call seeking comment. A Microsoft spokeswoman was unavailable for comment.