Ortlieb makes a few different backpacks. This one caught my eye, it's waterproof (like their panniers) and has a well-ventilated surface where it sits on your back. Not cheap, but its sounds like you prefer quality.

http://www.ortlieb.com/_prod.php?lan...kt=packmanpro2
http://www.ortliebusa.com/prodInfo.asp?pid=234&cid=2
(links are to the same backpack, one in German, one in English)