Enter Michael Schwarz's AJAX .NET wrapper which allows ASP.NET developers to quickly and easily deploy pages capable of easily capitalizing on AJAX. Users should be cautioned that the wrapper is early in development, and as such isn't fully matured.
It should be pointed out that technologies such as AJAX are very likely going to lead to violations of layered architectures (N-Tier). My opinion is that AJAX increases the likelihood that the presentation logic layer (or worse, the business layer) will leak into the presentation layer. Strict architects, like me, might cringe at this notion. I feel that even if AJAX is used in a manor which slightly violates layer boundaries, the payoffs are well worth it. Of course, that's something you'll need to look at for your specific project and environment.