Igor Kromin shared this project on his blog. Amazingly, you can control the original system's power button, indicator LED, and even integrate the original controllers using an open source driver and the Raspberry Pi's GPIO pins. You'll need to do some light soldering to make this a reality, but once you're done, you can run retro NES games through an emulator on the Pi, and even load up new games through a USB port exposed in the cartridge bay.
If you're just interested in emulating old games on your Raspberry Pi, obviously you don't have to go through all this trouble. But it is a really cool way to preserve gaming's past and impress your friends.
Raspberry Pi in a NES Case - Part 1 - Wiring up the front panel and external connectors | Igor's Blog