http://gavintech.blogspot.com/2012/05/neutralizing-51-attack.htmlFrom Gavin's blog:
Something like "ignore a longer chain orphaning the current best chain if the sum(priorities of transactions included in new chain) is much less than sum(priorities of transactions in the part of the current best chain that would be orphaned)" would mean a 51% attacker would have to have both lots of hashing power AND lots of old, high-priority bitcoins to keep up a transaction-denial-of-service attack. And they'd pretty quickly run out of old, high-priority bitcoins and would be forced to either include other people's transactions or have their chain rejected.
It sounds like a simple thing to do, but has it been implemented yet?
It might sound simple to do , but it really isn't that simple .
As i know , it is not implemented yet ..