The Osprey Momentum 34 should handle all your needs.

1. Integrated pull-out rain cover
2. Side pockets for water bottles.
3. U-Lock storage in outside stretch pocket.
4. Awesome phone/MP3 pocket on shoulder strap.
5. Laptop compartment is big enough to hold a 17" Macbook Pro

It has a ton of other bike commuter specific features also. Perfect for a short bike commute, and a lifetime warranty. Available in Bamboo or Carbide.

http://www.ospreypacks.com/en/produc...te/momentum_34

Here's a link to the owner's manual:
http://www.ospreypacks.com/en/document/download/155