I've been looking for the same thing - I've found something similar to what GWT does for ASP.Net called Script#. It is written by the same guy that was mainly responsible for ASP.NET AJAX - Nikhil Kothari. The site is: http://projects.nikhilk.net/ScriptSharp
It basically takes C# code and compiles it to Javascript - the same thing that GWT does (only it does it with Java)
I honestly think that GWT can work with ASP.NET and even ASP.NET MVC - I reckon that you'd need JSON or XML serving generic handlers (.ashx) in ASP.NET for GWT to work. I havent tried it myself..
Forgive me if I've confused anyone..
