The big question however..
Is how many of those 25m are replacements for failed units? I have had 3 failied Xbox360's. 1 of them replaced by Microsoft, but 2 of them swapped by the retailer, as they failed very early on in their life. When the retailer replaced them, they went through the till as a new sale.
I bet there are only about 20m ACTIVE Xbox360's out there....