RIM is so far behind the curve with their Blackberries that it isn’t even funny.
iPhone or Android. These are the platforms with significant 3rd party app support. As an example, the popular Sonos music system has support for iPhone and Andoid. No Blackberry, no Windows phone.
Windows has about the same amount of apps that Android did in their first year. From what I’ve read, it’s reasonably expected to get more apps quickly than Android, in the same time.
What Windows could really do to get a jump on both Apple and Google is if they used their lack of apps to organize their marketplace so apps are easy to find.
I really hate just looking under a large category and have to root through 100 pages of apps to find new ones. Like most people, they search through the first three pages and choose from there.