Ripple's consensus algorithm pretty much has this licked.
https://ripple.com/wiki/Consensus - http://bitcoin.stackexchange.com/questions/10180/what-are-th... - http://bitcoin.stackexchange.com/questions/13330/what-is-the...
https://ripple.com/wiki/Consensus - http://bitcoin.stackexchange.com/questions/10180/what-are-th... - http://bitcoin.stackexchange.com/questions/13330/what-is-the...