Bitcoin to Amazon's new "currency" or other virtual currencies like e-Gold, or Second Life's Linden dollars is like comparing gold to gold paint.
Sure, some people might value gold paint and use it on certain things, but it's unlikely to ever have widespread monetary value like gold has. The reason is anyone can create gold paint, there is no limit to how much they create, and the creation of it is done centrally. If a government wanted to clamp down and stop them manufacturing the gold paint they could do so since they know where to find them. This is why eGold and Liberty Dollar got stopped by the Feds.
Gold on the other hand is not centrally controlled. The value it has can't be squashed out by any government because people all over the world will still value it. Bitcoin is similar to gold this way. Note I didn't say Bitcoin is gold. I'm saying it has similar properties to it (by not being centralized) unlike other centrally made currencies.