Jon - In my back pants pocket, I carry a simple, thin wallet filled with about $10-15 in various currencies and a few "credit cards." The credit cards look real but are actually a collection of old cards from movie rentals, memberships, and credit card applications with our named scratched out. I keep enough money (including USD) in my wallet to satiate a thief, but the variety of currencies would make it appear like there is a lot of money without allowing for a quick conversion.
The idea is that my fake wallet makes an obvious target for pick-pocketers while keeping my real valuables safely hidden in my money belt and hidden zip pockets. Similarly, if I was ever held-up, I would happily hand over my fake wallet to satisfy the thief.
I got this idea from a friend who was robbed at gun point in San Jose, Costa Rica. It was her first day in the country, but luckily she had a fake wallet to hand over. Unfortunately, her friend who had been in the city for three months, lost her purse, which contained cash, credit cards, a passport, and camera.
We went through some really sketchy areas and very crowded markets throughout Latin America, but never lost a dime!