Think about it this way though -- if you have a 51% attack against Bitcoin, you have to make fundamental changes in Bitcoin. If you have a consensus breaking attack against Ripple, you have to remove the conspirators from your trust list.
There is a reasonable fix for this though, just set a reorg limit say 100 blocks, and prompt for manual intervention from the user. Given a prolonged network partition situation, there would be either a central or democratic procedure to determine the winning branch, via a checkpoint. This assumes that network partition does not happen too often for longer than a day in practice.
Actually this might be the approach ppcoin eventually adopts.