Messenger style bags just creep me out, plain and simple. I don't think it's good on the back and shoulders to be carrying a load so asymmetrically!

I just bought a Trans Alpine 30 cycling specific backpack by Deuter and I'm in love with it. It's got a built in yellow rain cover, good venting, hydration sleeve, helmet holder, and it's big enough to carry a laptop, school books, and commuting stuffs:

http://www.deuterusa.com/trans30.html

They also make some very nice smaller backpacks with cycling in mind. If you're in Portland, River City Bicycles carries the TransAlp30 and I've seen the smaller ones at Bike Gallery.