I was at a trade show for outdoor retailers last weekend and was able to compare a lot of bags from Chrome, Timbuk2, Crumpler and others, as possible replacements for the Nike backpack I now use. I sometimes carry a 12 1/2" laptop and often a lot of bulky color cards, samples, etc for customers.
I ruled out the pure messenger bag styles for lacking much more than one main large compartment. The heavy vinyl material also turned me off. I was interested for a while in Chrome's Soma, a laptop dedicated bag with built in sleeve that will hold up to a 17" Powerbook. Then I started questioning the heavy metal buckle and the contoured strap which would only allow the bag to be used on the left shoulder (can't alternate for comfort). But the decision maker was the poorly designed top that shifted open and inch or so when wearing it. This could definitely allow rain to enter and damage your laptop. The guys from Chrome readily admitted the design deficiency and suggested waiting for the next version due out this summer that would have this problem corrected.
Then I discovered the Commute from Timbuk2. They label it as, "Messenger bag style meets laptop computer protection." It has a cordoroy lined laptop sleeve (the medium holds a 15" powerbook and the XL size holds the 17" Powerbook) and a well engineered amount of storage pockets, water bottle and ipod holders, etc. The removeable shoulder strap has the same quick release system as the Chrome but without the heavy metal buckles. And, it can be used on either shoulder. The only drawback was that you had to purchase a shoulder pad separately if you wanted one. Also has a chest strap. The sales folks said it was their best selling bag for commuters. Haven't got around to ordering it yet, maybe this weekend.
You can see it here:
http://www.timbuk2.com/tb2/retail/ca...m?categoryId=7
Note that they give a good comparison to their Laptop Messenger bag under FAQ's.