Well, a 360 running a demo with reduced (throttled back) hardware specs & no method of interaction.
Presumably there are no final production model Xboxes and/or games ready yet; I'd imagine they'd have put the actual things there for playing and whatnot if there were (it'd make no sense not to).
I've read the final devkits are scheduled to go out around July-time (which'd give what, 4-5 months to optimize & test the release date games?); so I'd imagine it'd be the next generation that actually make full use (because you, IMO, couldn't develop 'full-strength' games on the basis of a 1 or 2/3 power devkit without the risk of under or overestimation of resources). Likewise for the Revolution/PS3, of course.
Albeit I'm sure I read - before E3 - that the devkits were 2/3, not 30% power. i think in an interview with one of the guys from Epic, not sure. It's possible he was wrong, or even that Ms are intending on upping the specs as a response to the PS3 (despite Ms' own PR analysis, most reports assign the PS3 as being superior - whether or not this is true, I'd imagine MS will be preparing a more effective counter than media-spread techy stuff).