Sounds like a design mistake to me! Very nice catch, paul.gaastra. That picture with your multimeter definitely proves, IMO, that the silkscreen (and the capacitor placement) was wrong. As far as I know, and my EE coworker agrees, there should never be a negative potential across an electrolytic (or tantalum) capacitor. They've been known to make exciting bangs when installed in reverse.

http://en.wikipedia.org/wiki/Electrolytic_capacitor#Polarity

Now I'm curious about other Mac models' motherboards that might have capacitors on the -5V rail...