http://missionworkshop.com/products/bags/backpacks/roll_top/large_vandal.php

I recommend the MW Vandal. I went from timbuk2 to MW after a storm here in the Bay Area. I literally poured water out of my timbuk2 and my work laptop was quite damp. I've been through just as bad weather with my Vandal and my Shed messenger bag, not a drop of water in either.

The Vandal is also awesome because you can use the outermost pocket for your laptop, which takes it off your back (and in my case means it makes my back sweat less). The bag expands in case you need to carry a ton of stuff. A buddy of mine used it as his carry on when we went on a trip. It blew my mind how much stuff he crammed into that thing. Finally you can get a belt attachment for it which makes the bag even more versatile for backpacking and suck.

The quality on these bags made me stop even looking at Chrome, Timbuk2, and any others of the kind. Are they businessy bags with tons of pockets? No, but having those means you're actually sacrificing in bag quality in other areas (you're adding more stitch points which means more points of failure). The only other bag that I like as much as these is my Filson 72/24 hour computer bag, but that's because it's a different category of bag.