Martin Rayla has a great thread over on Treasure Tables about using music in your game. He's not talking just about having something going on in the background, but also crafting a soundtrack that matches the expected actions, fights and drama that the players will be experiencing.

I've used music in my games before, but I've never made a concerted effort to sync up what was playing with the "on screen" action. That said, it's something I've wanted to do for a while, and our recent playtest of Battlestar Galactica gave me an excellent opportunity to try my hand at hobbling together a score of my own.