Palm, Microsoft and US mobile operator Verizon plan to hold a press conference today in San Francisco. It will probably be the stage for the unveiling of a Windows-based Treo smartphone, the subject of rumours on enthusiast websites for weeks.

The three companies did not offer any details about the subject of the conference in a press release on Friday, but the fact that they are bringing their top executives to the Palace Hotel in San Francisco tells its own story. Palm president and CEO Ed Colligan will be joined by Microsoft chairman and chief software architect Bill Gates and Denny Strigl, Verizon's president and chief executive officer, at the press conference, the companies have said.

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

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 OS, and handheld vendors actually shipped more Windows Mobile-powered devices during the past 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 at 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.

The Palm OS had been facing an uncertain future, with the purchase of PalmSource in June by Access Co., a Japanese mobile web browser company. PalmSource had said it was planning to turn to Linux as the base for future versions of the OS.

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 at Gartner. Palm choose 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 has 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 at Current Analysis.

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

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

One of the premier events on the mobile phone industry's calendar, the CTIA Wireless IT and Entertainment 2005 show, kicks off tomorrow in San Francisco.