After seeing all those rumors and official statements about the XBox Next it seems to me the device will be a lot closer to the GC instead of the XB:
-No HD
-Smaller size
-Proprietary disks
-IBM/ATI chips
...
Also, Microsoft has miscalculated some things and decided to market the XB to kids (their former enemies

) as well (official press release shortly after the release of Grabbed by the Ghoulies).
The XB2 is said to have three of those IBM processors, while the N5 will likely have two (DS was declared a "sample of what's to come with Nintendo's next console").
Perhaps they just envy Nintendo's bottom line?