right now i use a targus http://www.targus.com/us/product_details.asp?sku=TSB312 and it works great.

however am thinking of switching to messenger bag because of the whole 'easier to look behind' point made above.

plus, maybe i won't carry as much stuff causing my backpack to be so frickin' heavy

p.s. i was always wondering the aerodynamic differences between backpacks, panniers ... i guess drag is drag no matter what ...