Toothpaste is such versatile material. When I was in college, I used it as drywall spackle to fill in nail holes. As a kid at summer camp, I used it to write my name on the rafters of our cabin. I've heard that some people even clean their teeth with it.
Check out this tip from Lifehacker. It seems that a dab of Arm & Hammer Dental Care Advance Cleaning toothpaste can be used to rub the nasty wrist stains clean off of a white MacBook. A reader also suggests that a Mr. Clean Magic Eraser (which is free of abrasives) also works.
MacBook owners, rejoice! A cheap solution is probably in your bathroom.