You're looking for a sling backpack if you want to have both single shoulder and double shoulder modes. The Kata 3n1 series are a nice choice, I just received mine (although the large size) today and I'm very pleased with it from every angle. It's built very well, has a very simple way to access lots of gear in a sling config, and is very comfortable as a backpack with 3 bodies, 9 lenses, batteries, chargers -- the whole shebang. I would not normally carry all of that gear at once, but it's nice to know I can if I'm going on a long day trip.
Of note: all of the Kata 3n1 bags have a MIR on them right now. The large is only 70 bucks after rebate, and the medium is 12 bucks less. Plan ahead, if you think you're going to buy more gear, buy a bigger bag now -- you'll always be able to sell it later if you choose.
The only thing I'm still debating on is the tripod.