I have come up with an awesome way to clean seat belts (at least those that can be detached from the car easily.) It will require simple green cleaner, dish soap, and warm water. To prepare the seat belts remove them and spray with simple green (set it to foam if you can.) Then wipe them.
Then find 2 buckets. In one bucket, fill it with warm water, some simple green, and a little squeeze of dish washing soap. Take the belt and let soak for a few hours. After letting soak, remove from mixture and put in a second bucket filled with just water (this will serve to remove the cleaner from the belt.) Let sit in bucket for 10-15 minutes while agitating the belt. Remove belt from water and let dry. Your belt should be looking new again!
Here are some pictures of what these belts looked like before and after.
(this is kinda a before shot, i didnt take a picture of the cleaned belt, but this one was slightly dirty, you can see how it was a little brown)
Dirty cleaning mixture
Dirty Agitated mixture
After picture of the center belt. It was stained terribly, dark brown from cola stains and coffee stains. Now it looks new!