Eh, I like the idea of casino chips.
But they'd be terribly easy to just make more of, and would veery easily loose it's value.
Kinda like the approach Metro 2033 is going for, where amunition is currency. That way you'll have to think about what you're doing, you really want that sweet set of armor, but can you afford running into the wasteland with only one spare clip?
Never played that game but I agree that ammunition makes more sense as a currency than bottle caps.
Gold makes the most sense because I'm sure people would start mining, even primitive humans mined for minerals and gold.
I think there should be counterfeit in the game with the ability to create counterfeit currency, although if you get caught using them... Vegas is known for its organize crime...
Also I think there should have been the option to produce counterfeit bottle caps with the ammo press in The Pitt..