The ASP.NET AJAX Release Candidate is available here. A major change between beta2 and the RC is the switch from the Microsoft.Web.* namespace to the System.Web.* namespace. It's a straightforward update to exisiting code and should simplify things going forward into the next release of the .NET Framework. No other major changes are expected between now and the final release.
At this point, we are only taking high priority fixes and will release before too much longer. If you encounter problems, please let us know. And thanks to those who have provided feedback so far. It has helped us prioritize our work and while we haven't been able to include everything we would like for this release, I do think we have a solid release coming and am looking forward to adding more features in our next release.