The protocol is, while the domain system isn't. But then, you could say that about IPv4 and IPv6 - you have to get your allocation from somewhere, and your LIR gets it from its RIR and your RIR gets it from IANA. As long as you're using publicaly addressable computers, the system is in some way centralised. (And that's probably okay).
DNS is federated, which means a central authority has discretion over how it's distributed and to whom. You can put up your own server at any time, but nobody besides you will use it unless VeriSign (or whoever operates the registry for your TLD) agrees that it is currently authoritative for your subdomain.
FTFY. DNS is a centralized service.