This is me talking outta my ass, but I don't think I'm far off...
It's one of the big things that has hurt X360 showings so far....that X360 dev kits don't have hardware that well-represents the capability of the final system. PS3 dev kits supposedly house a 1st gen Cell chip and, I believe, SLI'ed GeForce 6800 Ultras. Xenon kits house dual-G5 PPCs and an ATI 850x card. Having development hardware that is MUCH closer to the final hardware is going to guarantee far better showings of performance in the early videos and playing demonstrations. One big clusterfuck for MS, it seems.
Someone who works on next-gen consoles could tell me if I'm wrong or not.