Most of the bags that camera store salesmen suggest as things that "don't look like camera bags" actually *do* look like they contain something expensive. Any slick black bag pretty much screams "I'm a rich asshole, please take my bag and my wallet" if you end up in the wrong place / time. Same goes for the crumplers; they look like trendy laptop bags or something.
For awhile my favorite system was a toploader zoom AW bag by lowepro plus whatever lens cases I needed (also from lowepro). The lens cases have velcro which lets you affix them to loops on the sides of the toploader zoom (which itself will carry a camera + long lens in ready-to-shoot position). You can carry this around by the shoulder when it's safe to look like you have camera stuff, or you can disassemble it and stuff it into a cheap green surplus-store duffel bag when you want to look like you're carrying your dirty laundry to the laundromat.
I used this setup all around Belize, Argentina, Guatemala, Mexico, and the USA without any problems.
Of course one thing to consider is that if someone is going to rob you it'll just as likely be for your wallet, but still.. in general I see no need to carry something that screams "thousands of dollars worth of camera gear!"