You can add the ASP.NET AJAX Control Toolkit to the list of products that has been updated to target Visual Studio 2008 and the .NET Framework 3.5.
The toolkit itself is hosted on the AJAX Control Toolkit page on CodePlex, Microsoft’s site for its shared-source development projects.
In addition, and not unexpectedly, Scott Guthrie has a blog post in which he announced the toolkit update and pointed to a variety of videos and tutorials. There’s also a bit of information about migrating Web projects to Visual Studio 2008 — as is evidenced by the blog entry title, ASP.NET AJAX Control Toolkit and Web Deployment Project Releases for VS 2008.
Finally, the .NET Developments blog’s Shameless Plug Dept. would be remiss in not referring readers to the ASP.NET AJAX Control Toolkit Learning Guide, which was published on SearchWinDevelopment.com back in April.