I have become enamored of new European packs which allow for complete back ventilation. I picked up one by Deuter in Amsterdam last fall and it has become my favorite day and cycling pack. Since I sweat a lot, I generally try and avoid wearing packs while exercising, but this pack has eliminated this problem. The new packs are suspended off your back with a nylon mesh fabric. This web also makes the pack the most comfortable I have ever worn, as weight distributes itself across your back, and the trampoline effect of the mesh absorbs the shock loads you usually get from packs while exercising.
I have found three companies that make them (interestingly all German). I am using one by Deuter called the AC Lite 15. A professional cyclist friend uses one of the small Vaude packs like this and swears by it as well. They all seem to have good allowances for hydration systems, and some have integrated rain covers and helmet holders. The down side is that because of the frame it is not the lightest pack you can get for its size, but even for an ultra-light weenie like me the trade off has been worth it. They are somewhat difficult to find in the US, but I have seen them for sale in some mountaineering and cycling shops. You can get catalogs from the websites and do mail order as well. Each company that makes them also makes traditional suspension pack systems, so inspect the catalogs carefully for the buzzwords like Air Comfort and AeroFlex suspension.