The C++ code is sort of dead, so it's either going to be Visual Basic or C#, depending on if you want to work on the current code or the future code (Darwinbots 3). Eric's maintaining the Visual Basic code, so he could probably help get you started there if you want.
With C# and Darwinbots 3, I'd really like to get someone other than me working on the GUI and graphics. It's an important step so I can start working on the physics, but it's a lot of work. I'll probably type up a post in the Darwinbots3 subforum explaining the different modules that need to exist.
Or if you're really ambitious you could help me work on the physics engine. The physics and mathematics involved is pretty intense, though, compared with most other things.