I found that a messenger bag rides low enough on my back that I don't feel unstable cycling with one on, and my back doesn't get hot and sweaty like when I cycle with a backpack on. Patagonia makes a nice bag called the Critical Mass, and Timbuktu makes nice looking messenger bags. The nice thing about wearing the messenger bag is that you just lock up your bike and walk, instead of messing around strapping your pack to your bike, etc.
Carradice and Ortlieb both make high quality single panniers designed for commuters with dividers and compartments for laptops, books, etc., but they are not cheap by any stretch of the imagination.