Are there any methods of "unyellowing" old beige PC parts?

Have a look at the retr0bright page.

“The problem was finally cracked in late July 2008 with a mixture of hydrogen peroxide, a small amount of an “Oxy” laundry booster as a catalyst and a UV lamp; we believed that this could do the job in hours instead of days. "

Forum support thread on AmiBay.

After and before:

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Pencil eraser does this. It's a bit manual and takes a while (depending on the size of the eraser employed).

Using a pencil eraser, you can write frustrated messages to IT staff who have condemned you to labor on equipment old enough to still be beige/yellowed.