Market Intelligence firm iSuppli predicts that while Windows Mobile's overall marketshare might fall in the coming years compared to its competitors, the dramatic expansion of the overall market could easily provide continually expanding real installed base numbers for the foreseeable future. They even believe that in the short term, Windows Mobile will increase its share to retake the #2 slot, though I question their numbers.
That being said, it's not all defections in Windows Mobile news LG is a relatively new Windows Mobile licensee, and they've got big plans, with 50 handsets on the planning board. And it's worth noting that Palm and Motorola, the two big defections that have made news, are maybe less important than they seem, with Palm destined to return to its own OS despite WinMo's strengths/weaknesses, and Motorola itself being in decline, separate from its smartphone strategy.
I think the biggest determiner of Microsoft's mobile computing fortunes will be what improvements are made to Windows Mobile in the upcoming version 6.5 and 7 releases. Truth is, Microsoft was flatly outmaneuvered by its competitors, and the current version of Windows Mobile neither supports the kinds of hardware, such as a multi-touch screen, that the latest smartphones must have, nor does its dated-feeling user interface hold up well against the iPhone or Palm, for example. But Microsoft seems determined to make up for lost time, and it wouldn't be the first time that they arrive late to the party and go on to dominate the market.