Niamh & SaintlyTurkey, you guys obviously aren't familiar with the earlier Fallout games so let me regale you on the history and reasons for Nuka-Cola bottle caps being used as a common/standard wasteland currency.
"In the Core Region, their relative scarcity made them a readily available currency for Hub merchants in the 22nd century. This lead to the nickname "Hubbucks", due to their widespread introduction by Hub merchants. The Hub merchants reasoning for their support was that the technology to manufacture bottle caps and paint the surface had been lost in the Great War, which greatly limits any counterfeiting efforts. Secondly, there is a limited number of bottle caps, which would serve to preserve their value to some degree. For similiar reasons, the East Coast merchants also recognize bottle caps as a currency, although who backs them and guarantees their value is unknown.
While their relative scarcity made them a readily available currency for Hub merchants in the 22nd century, although by mid-23rd century, they were replaced by New California Republic (NCR) dollars, which were backed by gold. However, shortly after in an ironic twist of fate, caps once again became a regularly-used currency when the destruction of NCR's gold reserves by the Brotherhood caused the value of a dollar to fall dramatically.
By late-23rd century the NCR's tenuous grip on the Core Region and the presence of Caesar's Legion (who have their own currency), as well as the low value of the now water-backed NCR dollar, bottle caps are the standard currency of the Mojave Wasteland in 2281, although NCR and Legion currency are both considered legal tender by the various caravan companies and on the New Vegas Strip. Mojave merchants also accept nonstandard variants, such as Sunset Sarsaparilla bottle caps."
Ref: http://fallout.wikia.com/wiki/Bottle_cap
Basically, there was just regular goods bartar until the town called Hub was established in former California but merchants. Bartaring goods for goods like livestock and other larger things became too time costly and cumbersome for trade affairs, which lead all the head merchants within the Hub to get together and collectively decided on a standard form of currency to make transactions much more efficient and smooth. They decided on caps for the following reasons:
"Bottle caps are backed by The Hub merchants in the Core Region. The reasoning for their support is that the technology to manufacture bottle caps and paint the surface has been lost in the Great War, which would greatly limit any counterfeiting efforts. Secondly, there is a limited number of bottle caps, which would serve to preserve their value to some degree.
Bottle caps were also called Hubbucks, because they originated from the Hub. "
Ref: http://fallout.wikia.com/wiki/Bottle_cap
Well, that's it for the history lesson today boys & girls. Hopefully that was somewhat insightful. Although, I'd like to add that using useless bottle caps as currency really isn't much different than using useless paper notes as currency like our money today, but the reasons justify the means. However mind you, bottle caps most likely aren't accepted continent-wide. Only in select places like on the west coast and somehow over on the east coast. Everywhere inbetween (for wherever is safe from the radioactive storms hammering the central land of the continent) is unknown what is used as currency if there even is one at all besides bartar.