This is where MS can get the upper hand. If they are open enough with their app store, they can get developers there as the developer experience is going to be very good.
Its a chicken and egg thing though, most serious developers won't code for it until there is a market. Though they will get a fair few .NET developers who haven't done any Android or Objective-C iPhone stuff.
Yes and no. I have to admit I hoped for the ability to install any app you like (like in former versions of WinMo). Sadly that is no longer the case.
Silverlight/XNA development is fully OK for me, but it seems a tad restricted. Especially without the ability to run Silverlight in the browser, something which really, really surprised me when I heard it (at least that's what I got told on the Windows Phone 7 developer session).
Its a chicken and egg thing though, most serious developers won't code for it until there is a market. Though they will get a fair few .NET developers who haven't done any Android or Objective-C iPhone stuff.