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.