Palm Inc., Microsoft Corp., and Verizon Inc. plan to hold a press conference Monday in San Francisco, which will probably be the stage for the introduction of a Windows-based Treo smart phone that has been rumored on enthusiast Web sites for weeks.

The three companies did not offer any details about the subject of the press conference in a press release Friday. However, the companies are bringing their top executives to The Palace Hotel in San Francisco on Monday. Palm President and Chief Executive Officer Ed Colligan will be joined by Microsoft Chairman and Chief Software Architect Bill Gates and Denny Strigl, Verizons president and chief executive officer, at the press conference, the companies said.

Ever since Palm separated from PalmSource Inc., the company that develops the Palm OS, 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 (http://www.engadget.com) 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 Inc.

Microsofts 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 Inc. 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 Microsofts products, he said.

A Palm spokeswoman did not immediately return a call seeking comment.

One of the premier events on the mobile phone industrys calendar, the CTIA Wireless IT and Entertainment 2005 show, kicks off Tuesday in San Francisco.