Anyone can 'split' the DAO at any point of time, which renders the 51% attack moot. A 51% attack works only when the attacker can get more than what he puts in.

Consider this obvious attack vector: Let's say today is the last crowdfunding day, and the DAO has raised $150 million so far. A wealthy attacker can immediately put another $150 million, effectively controlling 50% of the voting for the DAO, and then vote to send all the money to his own Ethereum address. This is perfectly possible, since this is all written in Ethereum contracts anyway.

To avoid this, the rest of the people can 'fork' the DAO, effectively leaving the attacker with 100% control over his share of 50%. The attacker gains nothing. The other 'fork' can now continue on.

This forking described above is also going to be used (I think) when it comes to funding new projects. I am sure some people will disagree with some proposal, and they would fork the DAO away to not invest in that proposal even though the majority agreed they would. They are free to do so.