I think that is pretty cool, and I agree with making sure you have full credit. I can't see a downside, but I could just be shortsighted.

It could be a crappy game you don't want to be associated with.
Phhttt. But who cares?
Rock on and onward.

There doesn't seem to be any downside unless you think there is a chance you could sell the song later - that might be problematic if you've already given some or all rights away. Make sure the approval is limited to use within the game (be very specific) and don't assume that you can rescind permission later - if the game has already been distributed, the octopus is out of the string bag and you know how that goes.

What eth said. Check to make sure the game is not something you would not want your name attached to.

I've donated computer graphics to an educational game and to an independent short film. There were really no down sides. I got credited both times. I have a listing on IMDB. There was no promise of money. Both were requests that I volunteered for.

I second the checking of making sure that the game is something that you don't find repugnant. If your name is attached to it permanently, and it will be if you agree, that will stick.

In my long run, it helped me to build my portfolio and to get some notice. I changed directions years ago, but my name is still out there.

"Hi Dan, thanks for your willingness to be involved in this project. but it turns out I won't be needing "Sunburst". Please keep making wonderful music, I love listening to (and reading) your Metafilter contributions."

Ah, fame is fleeting!