No, it isn't in any way similar to Bitcoin's blockchain consensus - that's is designed to have blockchain disagreements which are resolved eventually by some of the nodes rolling back their blockchain and moving to the consensus one. As I understand it Ripple doesn't do that - once nodes have agreed on a ledger version they won't roll it back, and even if they did the OpenCoin Inc-controlled nodes don't care what non-OpenCoin-approved nodes think when forming their consensus.
So the net result is that, since everyone - including the exchanges - trusts the OpenCoin-run nodes, the only safe thing to do is wait for OpenCoin's servers to come to a consensus amongst themselves and then accept their ledger updates in their entirety. There is no way for anyone outside of OpenCoin Inc to influence what gets included in the ledger except if you allow it.
All the servers are agreeing with each other. You could just as well wait for any other reliable servers to come to a consensus amongst themselves and it would work just as well.
It is true that today we control the majority of validators that other important servers trust. We're working now on increasing the number of validators because that will improve the reliability and robustness of the network. We absolutely do not want people to have to trust us, or even think they have to trust us, so this is a real priority for us. Open sourcing the server was, obviously, a step in this direction.
So the net result is that, since everyone - including the exchanges - trusts the OpenCoin-run nodes, the only safe thing to do is wait for OpenCoin's servers to come to a consensus amongst themselves and then accept their ledger updates in their entirety. There is no way for anyone outside of OpenCoin Inc to influence what gets included in the ledger except if you allow it.