I’m building apps on top of asp.net 4.0 ajax preview 4, and it’s pretty cool. The name is a bit of a misnomer, since you can use all of the client-side ajax functionality without asp.net 4.0 at all. In fact, I’m using the client-side templating abilities by calling in to an asp.net mvc app that returns json results. Here’s a link to the preview bits.
I’ll have more up in a bit, as soon as I can write a demo for it!