As far as I can figure out, Symbian is effectively dead, thanks to arcane programming model and a perception of shrinking market.
Microsoft is very strict with the hardware spec on WP7, making it less attractive for 3d party manufacturers than Android. However, Microsoft has traditionally taken good care of developers, and it's not at all unlikely this will come to their rescue, especially if the Android platform gets too fragmented.
It's also interesting to note that the various platforms have different preferred languages:
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Microsoft is very strict with the hardware spec on WP7, making it less attractive for 3d party manufacturers than Android. However, Microsoft has traditionally taken good care of developers, and it's not at all unlikely this will come to their rescue, especially if the Android platform gets too fragmented.
It's also interesting to note that the various platforms have different preferred languages:
WP7 - C# (effectively - it's Silverlight/ .NET)
Android - Java
iPhone - Objective-C
Symbian - C++
Blackberry - Java
Looking at that list, I'd bet on Android, Blackberry and WP7.