Yellowing, as has been mentioned, happens as some sort of chemical change within the plastic itself and unfortunately can't be "cleaned off." I have a Cyber Lead that was mint when I got it and has yellowed in the two years since, despite very little exposure to sunlight and no exposure to fluorescent lights. I guess my only options are to live with it or paint it.
As for general cleaning options though, this might sound extreme, but I had a Windy that was so filthy that I took it completely apart and took it to a car wash. I tried to wash it by hand inside but it was just not happening. In the end it wasn't really that hard to take out the monitor, power supply, etc. I took out most of the wiring, but left some in places where it looked like more trouble than it was worth. Then I just brought the cab to a car wash in giant pieces and sprayed the hell out of them with the self-wash soap and power sprayer. I drained the thing out and let all the wiring (and molexes) dry out completely before reassembling, and it worked just fine!
You can see some before/afters here:
Before
After
Incidentally, be very careful if you use the Magic Eraser. It is essentially fine-grit sandpaper and WILL remove more of the finish and gloss than you want if you're not very careful. I only use it as an absolute last resort.