DIsclaimer: You may experience some in-consistent behavior using this technique. If so please let us know.
I have heard a lot of questions and confusion regarding web.debug.config and web.release.config. For example here is just one question on StackOverflow. The question states:
First let me explain, as I did to that question, the purpose of...
When working with different versions of AjaxControlToolkit, Visual Studio 2010 contains some interesting improvements compared to previous versions of Visual Studio.
When adding a particular version of AjaxControlToolkit controls (or other similar third-party controls) to the toolbox using the Choose Toolbox Items dialog, we now show...