For the first time Palm announced it will be putting Microsoft’s Windows Mobile 5.0 operating system on its 3G-enabled Treo handheld.

The news reflects not only Microsoft’s increasingly powerful role in the mobile operating system space – a space at one time owned by Symbian, Wind River, Palmsource et al – but also the growing competition in the wireless email and web segment where RIM, Intellisync and Extended Systems have been making hay.

Under the three-way deal announced yesterday, Verizon Wireless will start selling a Windows-based Palm device for use on its EVDO network in the United States. It will be Palm’s first 3G-enabled Treo.

“In our view, every professional will have a phone that connects up to their email,” said Microsoft’s Gates. “They’ll have a phone that works super well with Exchange and Outlook and all of Microsoft Office.”

The ActiveSync features of Windows Mobile 5.0 allow phone-Outlook synchronization and, when used with Exchange 2003 Service Pack 2, push email along the same lines of systems offered by RIM and Intellisync.

“Verizon… is going to have period of exclusivity on this product,” Palm CEO Ed Colligan said. “We certainly intend to bring it to other networks and other network technologies over time, but it will be some time into the middle of next year before that happens.”

Colligan said Palm will continue to sell existing Treos that run the Palm OS, developed by PalmSource, formerly a unit of Palm, but he didn’t sound very enthusiastic about it. “It’s really not a major change to that relationship,” he said.

“Sure there’ll be some cannibalisation [of Palm OS sales],” he added later. “There will be people who will want to move to this platform. No question about that. But I think it expands the market too. I think it gets more people seeing Treos who want to use this type of functionality.”

Intellisync, meanwhile, currently provides the email software that underlies Verizon’s consumer email services, and the company insists that this will still be available, even recommended, under Verizon’s forthcoming Windows-based service.

But when Microsoft launched Windows Mobile 5.0 earlier this summer, it said its focus on developers and the fact that millions are already familiar with the Windows interface would be the keys to the software’s eventual dominance. Going on yesterday’s news, it could just be right.