"One could argue that a block chain fork could invalidate an arbitrary number of blocks at any time, and therefore the Bitcoin protocol does not by itself have

"One could argue that a block chain fork could invalidate an arbitrary number of blocks at any time, and therefore the Bitcoin protocol does not by itself have this property of being able to tell which transactions really happened." No, one could not argue that. The economic majority will decide which fork is valid. That's the opposite of centralization. "To limit the number of blocks that could be invalidated in this way, the bitcoin reference implementation contains checkpoint hashes. These are indeed decided centrally. If you accept the core assumption in bitcoin that an attacker will never control a majority of the hashing power, then there probably isn't a problem and the checkpoint s aren't needed." You fail to realize you are not forced to download anything you don't want to. And the checkpoints are hundreds of blocks deep. They don't decide anything the network hasn't already decided (6 blocks deep transactions are considered practically irreversible). The checkpoints ARE needed as an anti-DOS measure (they protect the storage of full nodes from being flooded by forks that could suddenly start to appear at low block numbers). I don't know why people are so eager to spit opinions on things they don't know anything about.