+1 for timbuk2, though I share your discomfort regarding messenger bags their eventual slide into the armpit. timbuk2 bags have a strap that prevents this (mine fell off long ago, but I've noticed I can get a replacement, so I think I will)

if you're carrying much more than oh, 10 lbs (say, a laptop or college textbooks) you're probably better off with the rack/panniers. I don'd mind taking my laptop in the mb, but I find more and more I just leave it at work 2 or 3 nights a week to make the hills easier. (ok, well, putting it on a rack might not make hills much easier, but at least more comfortable)