I was thinking about it. I don't think it will be the 720, because what does that imply? I think part of the reason they went with 360 is as others have said, because Xbox 2 doesn't sound as advanced as Xbox 360 next to a PS3.
The best I could come up with as it would not only do 3D, but also with the new hyper Kinect it would move into your dimension, as you are part of a 3D game space - thus "Xbox for the 4 Dimension" thus "Xbox 4D" or "Xbox Cubed."
But honestly, 'Xbox Infinity' sounds better if you don't think of a car.
Plus Infinity rhymes with 360. (But I'm sure Apple would laugh as you could abbreviate it as an Ixbox. As in - want to go play my Ibox?)
PS one for fun, "Introducing the Xbox Atari!"