Actually, have you guys played NFS on one of the less high tech cell phones? I imagine it would work on the Zune if it could work on crap phones.
There's also a ti-83+ game I used to enjoy that is exactly what I'm talking about. Here's a screenshot
It's not 3D, but it has a neat perspective that makes it looks pseudo-3d, which is probably good enough for a game on the Zune. I'm not quite sure how it's done, but my guess is your car stays on the bottom of the screen while the background and environment scrolls through.
Now, I haven't looked at what's possible on the Zune given the apis and all that, but it definitely seems viable.
Again, if it can be done on a calc and cell phones, it can be done here.