I've been spending some time recently playing with Microsoft's Atlas framework. So far I've been pretty impressed with Microsoft's AJAX toolkit. They've come a long way toward building a capable framework that includes both client-side scripting and a client-side/server-side integration model that eases the transition for existing ASP.NET apps into the world of AJAX, while leverage a lot of the existing ASP.NET tooling.
Most of my time so far has been spent learning the client side of the framework, which layers some of the .NET framework's class and component models onto the Javascript programming language. It's an interesting approach, which has its strengths and weaknesses, but at least should help shallow out the learning curve for an experienced .NET developer.
Hopefully I'll have time to blog some of the things I've learned in the process.