I usually carry the F1x with 2 bodies, 24-70, 70-200 2.8 IS, 100-400, and 100 L as well as 1-2 flashes and extra cards. Domke and ThinkTank make backpack straps for use when you don't need immediate access to the bag (in airports or train stations, for example) but you've still got a shoulder bag with easy access while you're on the go. Plus you are less likely to have to check it at sites that don't allow backpacks, since it's a shoulder bag. My wife had to check her backpack-style purse at St. Mark's in Venice while I was able to keep my (bigger) Domke F802 with me since it's a shoulder bag.
good perspective. since i still have my canon over the shoulder bag, i think adding a backpack makes sense. going to order one right now 
