I'll second the Ortlieb recommendation. I've been using an Ortlieb Messsenger backpack on account of a separated collarbone (I had a thread going a while ago -- a year or so). Over 35+ miles, a messenger bag wasn't quite cutting it and so I opted to try a backpack out. It's made a world of difference.

I also tried out a regular North Face and a Mammut backpack (daypacks, if you will) and those served just as fine (as Aeroplane mentioned) but I like the simplicity, waterproofness and durability of an Ortlieb. I also wanted something that was a bit larger and designed with a cyclist in mind.

One thing of note: I also rode with a Chrome Backbone for two days or so before returning it -- my shoulders are wide and my neck Rollins-esque and the Chrome backpack straps were a little too close to each other thus pinning my neck and giving me pain there. FYI if you have a thick neck or such.