I was in a similar situation and ended up with a messenger bag. I was unable to find a backpack that had enough space.
I got the Timbuk2 command:
http://www.ebags.com/product/timbuk2...uctid=10005651
With a little googling and some patience you can get it for $60-70.
There is plenty of room in the main compartment and the compartments on the front have enough space for smaller items. I especially like the bottom compartment for chargers and assorted computer accessories.
It is lined with a waterproof fabric which was another selling point for me.
I really like the checkpoint friendly feature of it. You can grab the bag and go even if the laptop compartment is not zipped up again.