I haven't seen anything mentioned about the power supply, but the connector looks rather odd. And the picture of back of the XeDK
mentions using the "correct power supply model", not the "correct power cord".
This would seem to make sense as it makes the unit smaller and cooler, and you don't need to make different models of the box for US/everywhere else, just supply a different mains adapter. An external supply on the original xbox would have made it easier to properly fix the faulty ones.
My guess is that the power supply has always been hidden out of display when the box has been demonstrated. Does anyone have pictures? I wonder how big it is going to be.