Sergey Vlasov, from Tabs Studio, has found a workaround to the problem of VS 2010 SP1 breaks add-in debugging targetting .NET Framework 2.0 that I posted yesterday:

Debugging add-ins for Tabs Studio in VS 2010 SP1
http://blog.tabsstudio.com/2011/03/16/debugging-add-ins-for-tabs-studio-in-vs-2010-sp1/

Thanks, Sergey.

On the other hand, apparently this problem debugging add-ins targeting something below .NET Framework 4.0 can also happen without SP1 according to this report in the MSDN VXS Forum (although the reports refers to a shared add-in for Office, not an add-in for Visual Studio 2010)