VS.NET debugging is still causing problems, especially if you're trying to debug AJAX enabled websites on IIS7. Scott Gu have a great blog post about this, however his advice *only* works on non-AJAX websites. As soon as one try to debug an AJAX enabled website the, all to known, security failure pops up.
So, what you need to do, in order to debug, is to move the Application/Website to the "Classic .NET AppPool" (instead of the default one) - This (for whatever reason!) fixes the problem and you can, once again, be a happy - debugging - ASP.NET developer! :-)