I have had my Sealline Urban Backpack for a while now and thought I would update my review from here:
http://robonza.blogspot.com/2009/11/review-sealline-urban-backpack.html
This bag is great, I have commuted in pouring rain and snow and the bag has kept all of my stuff clean and dry. I also have been using it for hiking and have taken it on some pretty long snow and rain hikes with the same success.
I purchased the small model and it is more than big enough for commuting. I carry a change of clothes, laptop, tools and it fits fine.
I recommend this bag to anyone who rides in a wet environment.