I already know C# and am familiar with XNA.
So it would make sense to develop for the Windows Phone 7, but would it be worth while to learn Java, get familiar with Android, and instead develop for Android devices?
Android has been around longer so it would probably have a larger user base. Is it really worth it to try to develop for one of the more popular mobile platforms. (Windows Phone 7 has been out for like a month?)
I haven't tried either one yet though. I'm also going to buy a phone, so this decision would probably influence the phone that I buy. (to develop/test on)