IGN Speculates that the 360's GTA IV Second User Content to debut 6-1-09.

[Squilliam]

Munkeh111 said:

Squilliam said:
^ Zune HD or not, its possibly some sort of crossover device. I also heard rumours they have a strict specification guideline for Windows mobile 7 so that could be the iPhone to this iTouch if you use the Apple monikers.

So every Windows 7 device would be able to play Zune games?

From what I've seen, I would have to say yes. Though I have no idea on the variability of the interface though.