I have the Tamrac Expedition 5.
I have it LOADED with gear including (2) 30D's...one with a 70-200 f2.8 IS mounted, then I have a 30D with a 24-70 f2.8L mounted (both with lens hoods reversed), I also have a 10-22, a 20-35, a 1.4x extender and the hoods for both of the non mounted lenses and a Sony DCR-PC105 video camera stored in the main compartment.
I also carry (2) back up units including the HyperDrive Space, and an Epson P-3000, a remote switch, multiple filters, 2 small pelican cases with my sensor cleaning goods, a few lens cloths, 4 batteries for the 30D's and 3 batteries for the video camera, 2 chargers, notepad, pens, and 6 CF cards in the other available pouches and pockets.
I just got back from a 10 day trip to China and that Tamrac Expedition 5 with ALL of that crap went through the scanners and fit nicely in the overhead compartment of the 4 planes I got on and off of going to and coming from China with no problem whatsoever.
The Expedition 5 is a KILLER backpack, comfortable, fits nicely and is fully adjustable, extremely durable and as you can tell holds a TON of crap. It probably weighs in at close to 35 pounds with all of that stuff crammed in it.
That was just for the trip to China however and is NOT how I normally have it loaded for day trips.