I found the below page which is the most informative I have seen about the yellowing of plastic computer and console cases. After reading it im thinking of at a minimum rubbing uv protection compounds over my computers. Another idea is to paint them in clear uv resistant paint. If you have used hydrogen peroxide to bleach your computer cases (I think it may make them brittle, but anyway..) then you should clear coat them to stop them going yellow again. Both to block UV and to block oxygen. It seems both oxygen and light attack the plastic. The oxygen attacks worse I believe after a bleach job. If we do nothing and just let them yellow more they will become so brittle bits will start breaking off during maintenance etc. At some point I may try this uv clear coat idea on a case that is already damaged so ok to experiment on, to see if it makes it look too shiny or something.
http://www.vintagecomputing.com/index.php/archives/189