Of course there are. The identities and "distributed part" has to do with the servers that keep record of things in Bitcoin. In fact, a few months ago an economist wrote an article (which was here on HN) that described how the protocol among the "bank" servers in Bitcoin can be broken if there is a coalition of other "bank" servers that choose deviate. This was, in Computer Science terms, one expression of a Sybil Attack.

You should check Wikipedia for the background on Sybil Attacks.