Yes, the initial paragraph of the bitcoin paper (bitcoin.org/bitcoin.pdf) says that as long as 51% of the network is not attacking the network (that is, trying to forge their own, new, transaction history) the protocol is safe. Even when 51% of mining power is concentrated in the hands of a group, there is still a lot of reverse engineering to do to try and inject your own blocks as history. So far I have seen no demonstrable way to do this. If anyone is aware of research in this area I'd be interested to see how far it has come along.