Advice requested: I'm searching for a waterproof backpack.

[MontyCarlo]

I'm surprised nobody has mentioned Ortlieb bags.

They are expensive, but they are very durable and 100% waterproof. Almost all the bike messengers around Seattle use them:

http://www.ortliebusa.com/cartgenie/prodList.asp?scat=3

They also have roll-top messenger bags with shoulder straps.

[KarateChick]

u could always wrapp it in saran wrap

Actually, one wet day I wrapped up mine in a couple of smaller white garbage bags & everything inside stayed quite dry although I got soaked. Hmmmm...got a few outright guffaws directed at me when I was strapping that thing to my bike though....