Pull-string backpacks are good for short commutes!
I was surprised today to find that my lanyard string backpack (the one where the cinch cord for the top are the straps) worked great for a short commute where I didn't need to carry much other than a netbook. Lightweight, compact, and looks like a shopping bag when you need to duck into stores. If you get the type that is mesh, it aerates as well!
I'm sold - these are great for lightweight commutes. (My main commuter bag is a true Banjo Bros Commuter backpack, and for heavy loads, Jandd panniers.)