Need a rear USB port for the PS3? Here's a tutorial [updated]
A few words of warning, though — this isn't a true rear port. It's a borrowed port from the front. Don't plug anything into the same front USB port that you're using for the back or the PS3 will freak out and cut the peripheral's use and need to restart. MacSolidWorks are apparently working on a new mod to put a second USB port in the back, but as a true hub instead of a pass-through. We'd love to catch wind of it when they finish!
[Thanks, Jasonuscg!]
[Update: Shifted some words around to give proper creds to MacSolidWorks]