You might also look at http://unitdriven.codeplex.com - it is a Silverlight and WP7 unit test framework/runner, that also has an add-in DLL for nunit/mstest so you can write tests and have the same test code run in .NET, Silverlight and WP7.
As long as you are testing non-async behaviors, UnitDriven is pretty compatible with nunit (we built it to run a few hundred pre-existing nunit tests). It does have its own model for async tests - again specifically designed so the same test code runs in .NET, SL and WP7.
