CS2D for the Zune HD?
CS2D is essentially a 2D version of Counter Strike. There are Terrorists (Ts) and Counter Terrorists (CTs) Ts try to plant the bomb/prevent hostage rescue. CTs try to defuse bomb/kill all Ts/recue hostages/kill all the Ts.
For navigation, tilting the zune would be to move around, and tapping anywhere on the screen would have the player shoot at the spot where he/she touched. Tapping with both fingers would reload. Two-fingered double tapping over a dropped weapon would pick it up. If in the bomb site, pressing and holding would plant the bomb. Likewise for defusing it. Being near hostages would indicate them following you.
I'm pretty sure that the HD's Tegra platform is powerful enough to play a 2D shooter. If possible, it would be cool to incorperate multiplayer between zune HDs. (I.e., players Vs. Players, Players Vs. Bots)
here's a Youtube video showing it.