The Xbox 180 rumour is currently just that. Some sources are saying it is a new handheld console M$ are working on to compete against the NDS and PSP, which I wouldn't imagine being too successful considering the markets already held by these two other systems and the developers they have on board.

The other is that the Xbox 180 is actually just a recent nickname for the original Xbox, showing it is only half as good as the Xbox 360.