A quick solution to this problem is to add the following to the "AssemblyInfo.cs" file of the AJAX Control Toolkit project (not your own web application project):
Of course, at the beginning of this file, make sure that you have
After you have made these changes, rebuild the AJAX Control Toolkit, and then your web application. Your web application should build fine.
Update: A later build of AJAX Control Toolkit now works with Visual Studio 2010 without modification.