Interesting you should bring this up, because my primary field is numismatics. It is true that you should never, ever clean a coin as a collector. Now don't get me wrong, ancient and medieval coins which have been under the ground or in the ocean for centuries will have been, in some manner, "cleaned" before they enter the collectors' market. But one must be extraordinarily careful to distinguish between mechanically/physically removing dirt or debris and chemical restoration from any serious oxidation or sulfide formation. Chemical cleaning removes the metal oxides -- it can never restore them. As such it is verboten to clean any modern coin.You guys know that old computers will end up being like old coins, if you strip the surface layer off nobody will want to buy them.
Keeping this in mind, I'm not really sure of the real reason computers, specifically platinum macs, get yellowed. It mac be for a similar reason as white house paint, or it may be for a similar reason as plastics. I do know that PVC plastics will form a thin layer of yellowing due to the UV light in sunlight. (Similarly it can form metal chlorides -- don't put your coins in PVC holders such as those intended for cards or paper products) This layer protects the underlying PVC from furthur damage and it can be removed. However, many plastics also contain cheap fillers, and I cannot speculate what happens to them.
Food for thought.