if you don't mind sporting 'old' USPS stuff, get a USPS Pro Cycling team backpack. It has a stowed raincover, but most of it is waterproof anyway. http://www.postalbags.com/source/2.htm
its even got a hydration option, helmet holder, 2 bottle holders, holds drafting tubes, lightweight, the list goes on. this bag has been a true gem compared to all the other overpriced crap, but i don't beleive they make them anymore. you could check around though. oh and i am hauling at least 15 pounds of books daily for 15+ miles at a time and it remains comfortable.