WalksOn2Wheels,

If you haven't decided on a backpack yet, or if anyone else is considering doing this, make sure you get a pack that allows air to flow across your back. The other trick is that the maximum height of the backpack is different on the bike because your head is tilted back - particularly a problem if you're wearing a helmet. I would also recommend getting a backpack whose dimensions are within the average dimensions for carry-on luggage. Get a pack with a removable hip belt, and if at all possible, sit on your bike with the full pack before you buy it.

The biggest problem, I've found, is that backpacks are designed for an upright back. Specifically, the weight is supposed to rest on your hips via the hip belt, but as soon as you get on a bike, that stops working. Adopting a more upright posture can improve comfort somewhat, as long as your back remains curved enough to adsorb road shock, but I haven't figured out a way to get the weight off my back and onto my hips.