subversion externals are your friends.
Actually that's exactly what I was looking for.It sounds like you want to better organize how resusable classes are being used in different projects?
I've always found it easiest to keep my code library in source control and then branch the needed pieces to the project that depend on it. That way if you update the class it can easily be pushed to dependent project. Likewise, if you edit the class for that specific project, you can merge the changes back to the original trunk.
Not sure if that's what you're thinking/asking for.