We should be worried even when some pool has 20% of the total hash rate, they can get lucky several blocks in a row and do a double spend. 51% just means they will be lucky much more often and can perform double spend with better probability.
So yes, we should be worried about centralized mining, and we are since 2009, hence p2pool exists, but miners are greedy and selfish, unfortunately that's human nature.
However, even 90% of the hashrate still requires malicious intent of the pool owners to perform double spends, and they will be obviously visible too, causing panic and destroying any value of that doubly-spent coins. There is very little practical sense in such an action, similar to some nuclear state blowing up all their nukes on their own territory - just because they can does not mean they would.
Still waiting for an official statement from cex, meanwhile I hope miners with a brain will switch to p2pool. And if you would want to protect your coins - sell cex contracts you have *now*, nobody but cex ever made any money on them anyway; and go buy a 1TH hasher pointed at p2pool. The power is in our hands.
Indeed, I just don't understand the mentality of a lot of miners. Why pile into a centralized mining structure when we have the ability to decentralize? This is bitcoin....
