From the Bitcoin wiki:
Proof of Stake is a proposed alternative to Proof of Work. Like proof of work, proof of stake attempts to provide consensus and doublespend prevention (see "main" bitcointalk thread, and a Bounty Thread). Because creating forks is costless when you aren't burning an external resource Proof of Stake alone is considered to an unworkable consensus mechanism.[1]
It was probably first proposed here by Quantum Mechanic. With Proof of Work, the probability of mining a block depends on the work done by the miner (e.g. CPU/GPU cycles spent checking hashes). With Proof of Stake, the resource that's compared is the amount of Bitcoin a miner holds - someone holding 1% of the Bitcoin can mine 1% of the "Proof of Stake blocks".
Link:
https://en.bitcoin.it/wiki/Proof_of_StakeWhat are the implications of this for pure PoS coins like NXT? And how will hybrid PoW/PoS coins like Peercoin manage this issue once their networks switch over to PoS?
Someone could conceivably create a malicious client for a PoS coin that spams the network with fork attempts. Has anyone attempted this before? If so, what were the results? If not, then what do you think would happen if it were attempted?
EDIT: As I understand it, it is possible to fork a PoW coin with less than 50 percent hashpower. Having >50% merely guarantees that your attempt will be successful but having less than this could still work although your chances of success drop dramatically the lower your percentage of the network is. If PoS works in a similar way (substituting hashing power with number of coins/coin age), then someone with only a small amount of coins (e.g. 5%) could fork the network simply by spamming the network with fork attempts, correct?