If you're still on an Atlas CTP and you're wanting to move to the latest ASP.NET Ajax Beta, you've got some work to do.  Here is the ASP.NET Ajax upgrade checklist you will need to follow in order to run under the new beta.  If you've done much with the JavaScript extensions (Ajax control extenders, etc.), you've got even more work ahead.  This whitepaper describes in detail the differences between the Atlas CTPs and the ASP.NET Ajax betas/RTM (don't worry - it's only 54 printed pages <g> ).