When creating .NET classes that will be accessible via COM Interop, you need to be sure to add a default constructor to the class. If you don't, regasm will end up generating the tlb without the CanCreate flag for that class, and therefore, when registering the tlb, those classes will not be registered.

Yes, this makes sense, and yes, this is covered in MSDN, but I thought I'd mention it here, as it recently tripped me up for a little while.