Aluminum flexes too...
"The problem with the plastic is it flexes"
I had an iBook G3 with logic board problems, and apple kindly replaced it with a powerbook G4 (12").
During its 3 year warranty it problem had 3 or 4 new housings put on. It was my primary computer at one point, and was used quite a lot, and I noticed it wouldn't sit steady on the table. This was because heat was causing the entire case to warp!
I don't know what material they should go for - in my experience both Al and plastic have their problems...