Seems like we somehow screwed up in making ASP.NET 2.0 sites work properly with IE Mobile on Windows Mobile 6 / 6.1 devices (mainly due to a change in the user agent string that this browser sends, but I won't blame any particular product team here...)

Anyway, if you run into this kind of problem, Jim Wilson wrote a great article on how to add the proper browser definitions (.browser files) to your sites that will make sure the HttpBrowserCapabilities are set correctly for the new devices. Maybe there will even be an official fix by Microsoft one day...