I sure as hope they do not call it a 720. Now X^3 would be an intersting name. I though microsoft miss out on the X^2 name which I though was just kind of intersting to use because it is such a common math term we see everyday.
Well the release of the 360 wasn't too far from the original Xbox (3-4 years?), but no I doubt we'll see a new Xbox for a couple more years. Not with the current state of the economy and stuff.