There are two versions of the 51% attack, and I have made proposals that would address both of them.
Oh, and by the way, neither of these attacks are anywhere near as easy, nor as useful as popularly imagined. And we really have read 100 threads on the topic.
I have seen other people claim that this attack is not easy nor useful, but when asked to explain why their argument crumbles. By the way, I saw this post from kjj in another thread:
No, nothing in this thread is right.
...
The scenarios involving technical manipulation are entirely founded on misconceptions. The network really doesn't work the way you imagine it does. Someone would need several orders of magnitude more computing power than the rest of the world combined to pull off a block chain manipulation, and it would gain them very, very little.
Several orders of magnitude more computing power than the rest of the world combined to manipulate the chain? Hmmm. One thing is clear, you only need 50% or less of the total mining capacity (supposedly currently worth 50 million dollar or less) to create lots of problems. And this would be the most expensive attack possible. Obvioulsy you could come up with more cost efficient ways to attack. Clearly, you dont need alien technology.