There is a very nice survey of operating systems for mobile phones over at
Linux Magazine. Obviously, they come from the viewpoint that Linux is in the best position for the future, but they acknowlege the strenth and improvement of Windows Mobile, and the leverage of PalmOS as an application support layer (providing a set of APIs) that happens to run on Linux in it's next incarnation as ALP.
There are a lot of unanswered questions, and a lot of work to be done. But the next generations of feature phones and smart phones are going to be very interesting, and capabilities should explode quickly after the current foundational work has progressed a bit further.