This may have been discussed elsewhere, but I'm not sure what the answer is - if someone owns 51% of the hash power, can't he just ignore the rest of the network, and only focus on his chain, thus earning more than 51% of the rewards (100% in fact)? Will he in fact earn 100% of the reward?
Also - is it possible that several attackers working independently could achieve this scenario? If two people have 34% each, won't one of them always out-race the legitimate chain? Of course, they can't both win, but for some period of time, won't the stronger one among them gain more than his fair share of block rewards?