You may want to try a True North Gear pack model: Serac or a Weatherproof Firefly. (http://www.truenorthgear.com/product_listing.php?path=0_1)

Both packs have a roll top water/weather barrier that has always kept my 15 inch laptop dry even in rainstorms on my bike. This pack rides on your hips instead of your shoulders and has a fun urban/hip appeal to it. They make red and yellow for firefighters and search and rescue...but black looks good and inconspicuous.

The biggest problem with messenger bag styles for me was how it carries weight. After a 5 lb laptop and a 2 lb survival kit those "one shoulder" bags always left me lopsided...especially on a bike ;^).

Good luck.