OK, we really need to choose an engine so that we can start getting stuff together. The list of candidates is here: http://wiki.willblackmore.co.uk/doku.php?id=engine Unless anyone has any more to add, we may as well vote from the list. I vote XNA because it's quick and easy, and the pipeline is solid enough to use without much addition.
If it's made in XNA, does that mean an XBLA port would be relatively easy (with the $99 fee)? Maybe I'm jumping the gun a bit but that would be cool. Secondly, I'd be interested in helping some way, although like several other people, my coding skills are non-existent. Perhaps I could help with the graphics/icons?
If you have issues with the suggestion that has been made, why don't suggest an alternative? SDL supports Linux and is C++.
I'd be happiest with XNA since I've done a bit of C# before. I've not spent much time with C++ but I'm willing to get stuck in.