Using hydrogen peroxide to reverse case yellowing has been discussed here before, but Slashdot today linked to some ingenious Amiga folks and their process using peroxide and Oxy cleaner:
http://retr0bright.wikispaces.com/
The photos at the link above are impressive. There's also a full recipe for making the cleaner, and a couple variations.
I haven't got much beige equipment left, and even less that suffers from yellowing, but there's at least one Atari ST keyboard and one of my last 68Ks (a Mac LC II with a Warp030 accelerator) that I'm going to try this on.
I saw this on a couple of the Amiga forums too; and I am also going to give it a shot; I have a TON of old 68K hardware that has yellowed; and some CBM stuff as well!
A sane person whouldn't be this excited about OxyClean and Hydrogen Peroxide!