I agree with Ian Sane. The "N5" has a certain ring to it. I don't see Nintendo keeping the "GameCube" name, especially since the Cube was not as successful as they had hoped it would be.
The number theory that some have suggested actually makes a lot of sense, which is why Microsoft should not call their next console the "X-Box2".
"N5" is simple and bit mysterious. I hope they come up with a killer design for the machine, something elegant. Although, Nintendo seems to like gimmicks - I just hope the new console is not in the shape of a number "5" HAHAHA.