If you want a bike-specific backpack, it is hard to find better than the offerings from VauDe, with their very effective ventilation system http://www.vaude.com/epages/Vaude-de...IN&PageSize=99.

However, if you are planning to mount it on a rack (which I perefer to a backpack any day), I'd say go for panniers that offer the ability to use a shoulder strap (or have a comfortable handle).