If you have a strong background in C++, you can probably help out best working with upstream MythTV. Of course this is the center piece for every Mythbuntu install, so any work you do upstream will highly benefit Mythbuntu.
Get started by loading git onto your system. You can visit Version Control Basics if you aren't sure how to do this.
Next visit http://code.mythtv.org/trac/ and read in the "Want to Help Out" section for how you can help upstream.