I was flying a lot years ago and wanted a travel bag to cover motor racing - I ended up with a Lowepro Mini Trekker. They still make the same thing:
http://www.bhphotovideo.com/c/product/279323-REG/Lowepro_1990310_Mini_Trekker_Classic_Backpack.html
Here's a (really horrible) picture of it loaded up:
I've had the thing for 15 years or so, and it's carried multiple bodies, lenses, batteries, flashes, dirt, paperwork, those stupidly large sprorts press credentials, photo vests (sporting events issue these ugly things, gotta wear 'em) and SNACKS too. Pack carefully, you can get a few sammiches and a few water bottles in there.
Now it's big, but not TOO big - you might grow into it. I have mine loaded tight...when I get my 70-200 I'm practically hosed for space - there are a LOY of loop straps on the sides to attach additional/temporary cases. When I'd rent a super zoomer, I'd strap it on the side of the backpack.
Meanwhile, that little thingie that Shivaswrath showed looks very promising for an ultra-portable kit, and the pickpocket thing is a little harder when it's on your hip, under you hand.
The warning has been made - you can carry the backpacks on a plane, fling them around pretty roughly (not TOO rough, though. Padding only goes so far), carry them over one shoulder comfortably like a college student like I did for years, hike until you drop (mine has surprisingly comfortable hip straps), slog through a deluge to the apex of the hairpin at the track at Long Beach in a pouring rain and pull out dry equipment and a poncho, and they will hold together like they're made of steel. I love Lowepro.
...but it's VERY hard to stop and see something you want to shoot fast, then get a camera into play. The holster, you unzip, yank, focus, "click".
I love my mini trekker. You want to carry a laptop as well, see Mike's bit, although I squoze (?) a little compaq in mine once (no padding on the back, though). I think the holster works best for travel. I have one of those too, but not as nice as the one shown here. How much is that thing?