Is Proof of Stake a hard-fork? Proof of Stake is a proposed modification to the Bitcoin protocol.

maaku claimed that it doesn't require a hard fork / alt chain.

Is this true?
It depends on what PoS system is considered (or not, since I think the answer is the same either way).

Cunicula's system is completely different from what we have now. There's no way to "half" switch to it since a block which is valid in the current system means nothing in his system, and vice versa.

My system uses a skeleton which resembles the current system, so in theory you could solicit signatures in parallel and let anyone use them or not as he pleases. As theymos says a fork will still occur once there's a disagreement between the two systems about which is valid; and anyway, I think the process of collecting signatures and embedding them in the blockchain requires enough rethinking of the network that it is meaningless to regard it as anything but a hard fork.

Of course, even if you make an entirely new alt for it, you can always provide a built-in mechanism to allow bitcoin owners to import or copy their wealth, to make sure the early adopters of decentralized digital currencies at large don't suffer from the advent of a new and potentially better alternative to the original Bitcoin.