If it's any help, my own Power Tower seems quite resiliant to yellowing - certainly much more so than my A1200/500s ever were.
For the record, I found that kitchen cleaner and a scrubbing pad seemed to work well. However, the yellowness returns quickly so depending on how ambitious you are I'd probably take the tower front, scrub it gently with some fine sandpaper and spray it with an appropriate colour to match the rest of the case before applying satin/matt laquer.
If you want a custom paint job then now is as good a time as any!
The other method I find that works well is to keep the case out of sunlight and away from tobacco fumes. As an ex-smoker I can report that nothing yellows plastic like residue from smoking!
