The single company running largest exchange, largest pool, largest rating service? Kinda flies in the eye of decentralized.
Well, it depends on how you define "centralized." Currently Bitcoin is 100% owned by the mining cartel. I think that among the "core developer" only Gavin isn't running a mining operation. So if Mt.Gox or other exchange succeeds at breaking into the mining cartel then Bitcoin will actually become more diversified.
You do understand that waiting is no protecting from a 51% attack right? The attack chain would be built in secret. It wouldn't be released until it is longer than the valid change. So a block rated quadruple AAAA would instantly be erased without warning by a longer chain and the transaction replaced.
In a Bitcoin client unrated blocks could never replace rated blocks. So the 51% attack as it is known now would be a threat only for those who don't trust any rating service: hard-core anarchists, people from the fringes of society, etc. For everybody else the rating services will be the defense. Some will buy the ratings online, some will periodically download the ratings in the public library, very much like the access to financial ratings is happening right now. There is whole spectrum of speed-of-access vs. cost trade-offs that are possible.
Obviously the current hard-coded values in the clients will need to change: 6, 150, 500, probably many others. Right now the trust spectrum is: 100% gavinandresen & fabianhjr, 0% all others. This should change in the future.