I bought the Osprey Raptor 6 which came with a 3ltr camelback waterbladder while it was on clearance at evanscycles, Ive used it to or three times in the past year during the summer when temperatures were 25-33'c here in london. The backpack is small enough to hold a few snacks and the usual pump, spare tubes and multi-tool. but thats about it. Its quite a light, compact and low profile peice of kit. A lot of the Raptor line of bags have been discontinued now though :/
If i'd had to recommend something. Id wholeheartedly recommend the Race EXP Air bye Deuter
Theres a metal frame or wiring at the back which along with the mesh, keeps the bag propped up almost completely off your back so that air can still circulate and you get less of a sweaty back
It has space for a camelback as well though I have never used mine with it. but I have used the same backpack on 25-33'c rides and commutes and it has been most awesome and i cant recommend it enough. Its only a 10+2ltr bag so you cant exactly take the kitchen sink with you but you'll be able to carry all of the important stuff and a few extras.