My Knog Frank Dog pannier/messenger bag has a padded laptop sleeve built in. It is sized for a 15" Macbook, but my Lenovo T61 fits pretty well. A 13" dell wouldn't be a problem. I use it as a messenger bag most of the time, but it has an attachment that makes it into a pannier. People always comment on it when I carry it as a bag because it actually looks really well made. And then I show them how it attaches and they are dumbfounded. As a pannier it actually isn't that great the way it came as it doesn't attach at the bottom in any way, it just hangs, but I use a bungee cord to secure it.

Knog