Recently I made VS add-in with C# in VS 2008 (.NET target version
is 2.0). It loads and runs both in VS 2005 and VS 2008 without any
problem.

HTH
Alex

I'm guessing that the problem with it being compatible with 2003 but
not 2002 was due to the fact that VS was compiling your assemblies
against .net 2.0 rather than 1.1... I can't quite remember the release
order but that could be possible.

With 2008 you can actually develop side by side with 2005 since you
can target versions of the .net Framework. In fact just open 2008
create a new solution then add all of your projects. You will have a
2005 solution and a 2008 solution file that you can use to run the
exact same projects and develop side by side with 2005 developers. The
fact that they both work and build means that you can litterally
compile the exact same assembly with 2008 as with 2005. I would have
no fear in upgrading if I was you!

Of course doing that will result in you not being able to take
advantage of some of the new .net 3.5 features. But if you were to
upgrade the entire project to 3.5 and 2008 it "should" still be
backwards compatible. If you're properly under source control the
worst thing that can happen is that you have to "revert" your changes
if it has undesierable results.