I think I’ve found the perfect bag for a bike commuter. I wouldn’t use it for bike touring, but for day-to-day commuting to work or school, it’s perfect.
The Woodward Convertible from North St. Bags is a very well-thought-out convertible backpack and pannier. Use it as a pannier while riding your bike, then open a strip of velcro and pull out the straps to wear it as a backpack around campus. Voila! The best of both worlds.