Too lazy right now to read all the replies, but in case anyone is interested, I use the Lowepro Mini Trekker AW.

A backpack that fits my RB67 Pro-S, 3 lenses (one attached), and plenty of accessories. The only things I have that I don't carry with me are the dual-release cable (I don't really like it as much as I thought I would. I just use a single cable), and the split-image focus screen that I've only used a couple times.

It was really cheap on ebay ($30 or so).