Half of the time when I'm traveling transatlantic I'm carrying not only my carry-on, but also a photo backpack, as I'm into photography and shoot a lot on the trips. Unfortunately, the equipment is quite heavy, and there's a lot of it, so the bag is an essential item. So, i'll list the three I used during the last two years(and still do, depending on the situation):
1. Lowepro Flipside 300
In my point of view, that's the ideal bag for a city run, as it can hold a fullsize pro-DSLR like Canon 1Ds MkIII, or pretty much any DSLК with a battery grip,and that is very essential for me, one or two extra lens(depending on the lens size and\or need for room for other stuff), and the good thing is that the compartment opens from the back side, so when you wear it noone can open it.
2. Lowepro Computrekker AW
Great bag, perfectly fits a Pro-size DSLКб a lot of extra stuff, and a laptop up to 13.3". used it a lot, as the bag itself is extremely light and durable. Tore out a couple of zipper pulls, though, but I was an abuser at that period, so it could've lasted more.
3. ThinkTank Streetwalker HardDrive.
This one is the best bag I've ever used so far. ThinkTank is not a very known and marketed brand as Lowepro or any other, so it's apparently not sold everywhere. The idea is that the company was founded by photo enthusiasts tired of those huge heavy and unpractical photo bags and backpacks. Almost every bag I've seen is great in terms of usage comfort, as well as functionality.
The streetwakler hardrdive is a big enough backpack to fit not only the proDSLR w/ 24-70(that's not the smallest standard lens around, and a heavy one too), a 15.4" unibody Macbook and 2(!!) 70-200 2.8 Lens(that's a big&heavy one), but it allows to fit lens of size equal to 24-105 (all lens referred to are Canon, I can't compare with any others as I didn't have any handy) inside not in horizontal, but in vertical position - the interior is extremely capable of storing lots of photo equipment. I flew with this backpack last time DME-MUC-SFO, and they weighted it in MUC - it was about 45 pounds(!). Happily, they weren't in a mood to force me check it.
The website of ThinkTank is www.thinktankphoto.com , in case anyone is interested. I might post a couple of interior pics, if I was asked to.