Is it 51% attack or 50.0000...1% attack?
Bitcoin is getting to a point where 1% is a lot of hashing power...
My understanding is >50% due to the nature of the protocol.
When performing a 51% attack, you will need to find new blocks faster than the remainder of the network combined. This way, your block chain will become the longest and clients will consider it the main chain.
Theoretically, this is achieved (on long term) when you have >50% of the hash power.
But as we know, a lot of luck is involved in mining. It's possible that someone with 55% or even 60% of the total hash power will only succeed in his 51% attack after many weeks because he has bad luck. (Note that with >50%, statistically you will always succeed on long term.) But it is also possible that someone with less than 50% can succeed on short term.