“OpenZDK” was recently released on zuneboards.com. This is a Zune hack that, among other things, allows for custom-made 3d apps, multiplayer apps, and makes it so you don’t have to restart the Zune after closing a custom app. This opens the way to lots of 3d game ports like Doom and I believe also makes it possible for gameboy emulators to be ported to Zune:jizz:. The first big app to be released using this is Doom. It is just the freeware version so it isn’t the entire game, but I think there is a way to buy and play the full version on Zune.
Here is an article about it: Zuneboards OpenZDK
Here are some threads including an installation tutorial and Doom download.
Zuneboards OpenZDK apps
Edit: pics
The screen in these pictures is a little small because they were taken before an update that allowed you to change the screen size.