The Tad Gear backpacks seem nice, but I see no mention of being waterproof or weatherproof on their website. My Ortlieb backpack has a special coating on it that prevents water from getting inside. It also uses Tizip zippers, which are waterproof zippers that are used on wetsuits. The backpacks made by messenger bag companies (Freight Baggage, Ortlieb, Seagull) typically have an inner layer of waterproof vinyl that prevents water from reaching anything inside the bag. Normal cordura is alright for a light shower, but I would be willing to submerge my backpack in water with my laptop inside it.

I've heard nothing but good stuff about the Pac Designs bags. They are supposedly the best. If you order one of the larger, more expensive bags you can also request the X-Strap option, which adds another strap to the bag effectively making it a backpack whenever you want.