I have had a variety of camelbaks over the years, and yes the sweaty back thing is a problem. Then I found the Deuter Race X Air on clearance at REI-outlet.com for $30 - it has an air web suspension thingy. The pack is held an inch away from your back by a mesh screen.

They are a little hard to find, but I'm a big fan of Deuter's bicycling specific packs.