AJAX support will only be limited in the coming version of ASP.NET 2.0. Some controls will support refreshes using this method but it is implemented on control level and as such it’s not something you can get your hands on yourself in a easy manner. So it won’t make it this time around but you can go vote for it on the MSDN page for ASP.NET 2.0.
I already cast my vote, you should too.
Disclaimer The opinions expressed herein are my own personal opinions and do not represent my employer's view in any way.