Service Pack 1 of Visual Studio 2010 introduced a bug when debugging add-ins in VS 2010 targeting .NET Framework 2.0 that I blogged about back in March. They were kind enough to contact me some months ago to try a private hotfix (which was successful) and yesterday they made it public.

The Microsoft Connect bug report that I opened was this:

http://connect.microsoft.com/VisualStudio/feedback/details/650694/vs-2010-sp1-breaks-add-in-debugging-targetting-net-framework-2-0-rather-than-net-framework-4-0

and the new Knowledge Base (KB) article with the hotfix is this:

FIX: Breakpoints are not hit when you debug a Visual Studio Add-in project that targets the .NET Framework 2.0 in Microsoft Visual Studio 2010 SP1
http://support.microsoft.com/kb/2591546