I have owned both the previous white 2.2Ghz and the current aluminum 2.0Ghz. My previous one was stolen, but I got reimbursed from my insurance agency minus a $500 deductible. I went out and bought the aluminum one on release day. It was well worth it. In fact, I would gladly have paid $500 to upgrade from the white to aluminum.
I can't believe that anyone who has used both the plastic and aluminum versions could possibly say that the aluminum has worse build quality. The screen quality the same but with LED backlight. The case is rock solid instead of the creaking, cracking plastic (I had my topcase replaced once already on the plastic version). The trackpad is amazing. The speakers are actually usable instead of the tinny crap in the plastic version. By far the biggest difference is the Nvidia video. Even simply stuff like the "genie effect" on the old Macbooks wasn't smooth. On the new one, all of the video effects are silky smooth. You can watch Youtube and other flash-based video without the fan coming on full blast (or in my experience, coming on at all). Heat is almost non-existent on the aluminum. The plastic one cooked my balls even with just light use.
Aluminum all the way.