This bounty shouldn't be too hard, once bitcoinj has been developed, porting over to C# shouldn't be toohard. Then a WPF GUI needs to be made, and some sort of way to download/send transactions over http needs to be developed.
There's also the fact that Windows Phone 7 doesn't allow background processing or multitasking. Whenever you switch away from the app it stops. You'll have to download newest blocks everytime you start it. Again the november 'mango' update should help with this as some form of background operation is supposed to be available.
I don't see any chance of your bounty being claimed if only a completely client side solution is allowed until at least mango becomes available in November-ish (earlier to registered WP7 developers possibly).