The Ogio All Elements Backpack is a top quality waterproof backpack that will carry both a 15" laptop and a tablet computer at the same time. It's roll top enclosure keeps the water out and your gear safe. The pack retails for $125 on the Ogio website, but new customers can get a 20% discount if they sign up for the email list. There's also free shipping.
This bag is marketed to motorcyclists. It's got incredibly sturdy construction, reflective printing on all sides of the bag and pads designed to increase airflow between the bag and your back. There's an outside zippered pocket for easy access to wallets or keys.
The roll top definitely keeps the water out and there are two zippered pouches inside in addition to the laptop and tablet sleeves.
The entire inside sleeve zips out so you can use it without the electronics storage. Removing the sleeve also makes it easier to pack with you gear.
The adjustable chest and waist straps come with a system that allows you to secure the loose end of the strap after you've adjusted it to your preferred position. The bag is made of 400D nylon and comes with a lifetime limited warranty against manufacturing defects.
Of course, this isn't going to save your iPad when you fall off your bike but there's no backpack that's realistically going to protect your electronics and still be light enough to be workable. This is a really nice piece of gear. It's going to last a long time, it's highly reflective without subjecting the world to a rainbow of neon colors and it's really comfortable. Definitely worth a look.