just received a Dakine Cyclone rolltop backpack (purchased from Amazon) - it's got enough room for my 5d2 with 100-400 and 7D with 400 f5.6 in Thinktank holsters attached to belt plus another bag with short lens, flash, filters, etc.
normally this pack would easily pass as a carryon if not overstuffed, but i've got one small-plane flight that might be an issue
only thing missing is a good way to carry tripod - if i don't figure something out i can hopefully persuade my wife