Why? Because everyone who owns some of this intrinsically worthless asset immediately has an incentive to start working on talking them up. Social news sites are the front line of this battle... write a story about how they're going to be the next big thing and submit it, and you can be guaranteed a bunch of upvotes from everyone else who owns some bitcoins. This story alone probably caused at least a 10% increase in the value of 'em.
Personally I'll buy some bitcoins the moment they become exchangeable for pogs.