I've just updated the .DLLs that are currently in my project from v2.1.0.427 to v2.1.0.446 and my project no longer builds. Has this library been tested with Windows Phone 7? It says that it has, but my project no longer shows any of the ESRI namespaces as any good. I have a video showing the build errors in action if anyone needs to see.
Could you share the build error with us? (video is not necessary - just copy/paste the text) Make sure you added the reference to the WinPhone 7 assemblies, and NOT the Silverlight assemblies. This is a separate download, as Jennifer linked to .