Also had the same problem, solved with the following:
* Remove reference to AjaxToolkit.dll
* Add new reference AjaxToolkit.dll (download from Codeplex)
* Remove reference to Microsoft.Web.Preview.dll
* Add new reference to Microsoft.Web.Preview.dll which was included with your Microsoft ASP.NET AJAX 1.0 Beta 2
You may need to install the Futures November CTP if you are having troubles running your controls (which worked before). I was successful after killing the aspnet_wp working process in the running processes.
Sincerely,
Timothy Grant Vogelsang