[iwr]

Even if an attacker can muster the computing power to overwhelm the current bitcoin network, these efforts would be very visible and traceable to a few sources. You could thus tell people are giving out fake coins.

However, given the lack of inherent value (or backing) in a coin hash, bitcoins can't be used as money, only as money-substitutes (IOUs with an indeterminate recepient). The Dollar itself is "backed" by the US government requiring taxes be paid in it.