Via Mark, I ran into an article of Daniel Larson talking about the SharePoint Ajax toolkit on codeplex.
"So now that ASP.NET AJAX has hit the RTM, a lot of SharePoint developers are looking at using Atlas in SharePoint. Officially though, it's not supported in WSS or MOSS. But that doesn't mean you shouldn't use it, there are just Atlas components that you should realize are NOT compatible with WSS.
But there *ARE* ASP.NET AJAX technologies that are compatible and that you should use. So here is my guidelines on using Atlas within WSS or MOSS. I've probably said this before, so this may be redundant, but wanted to lay out some guidelines now that everything has hit RTM. "
..
"The SharePoint Ajax Toolkit is available now in source code as well as the WSP solution package. I forgot to check in this morning when I built the release, so if you grabbed source this morning the code won't run against the RTM Atlas build (although the WSP will, that is the latest available.) Be sure to grab change set 16960 or newer.
Source code: http://www.codeplex.com/sharepointajax/SourceControl/ListDownloadableCommits.aspx
Releases: http://www.codeplex.com/sharepointajax/Release/ProjectReleases.aspx"
read more here