In summary, I'd suggest going for a cross-platform approach.
Symbian has by far the biggest number of users and has the largest choice for programming languages.
Symbian and Maemo will be running Qt in the near future, as well as supporting open python, open C, java etc etc etc.... (they also both have the Qt libraries available now)
I wouldn't put too many eggs in the iPhone basket. Your application would have to be monumentally good to be found and paid for by a significant number of people in the 100,000 items in their app store.
Android, don't really know anything about it. It seems like it could be a popular platform, its at least a real multi-tasking environment (unlike iPhone from app dev point of view).
Palm Web OS is insignificant at this time.
Perhaps the best solution in fact is to make your application web-based then you can just develop small apps which hook in to the web service?
