Microsoft released the next iteration of their AJAX library today, RC1. You can download it here. They've made a few breaking changes since beta 2, but nothing drastic. Mostly you'll just have to update your web.config file to reflect the renamed namespace and a few other things.
Unfortunately it looks like they did not fix the WebParts stuff which doesn't work with AJAX, such as dragging and dropping a WebPart into different zones while in design mode. Also, any 3rd party controls which referenced the old namespace will not work. ComponentArt's Grid control for example.