Very, oh very softly rub with a cloth. (I do stress VERY softly). In a circular motion.
Another technique I've heard work is to tap the back of the screen around the area of the offending pixel (this can be done on a Powerbook and iBook, but I don't know about the new iMac).
I had a dead pixel on my new Powerbook last year, thankfully it was over where the Hard disk icons were placed. Apple's and every other manufacturers line is... 'they are within specification'. Which means, 'nuttin ta do wit us my friend!'!! Apparently within specification can be a very drastic amount (something like 20 per sq inch), I don't know whether this is true or not!