The zero confirmation branch is private, but basic integration into branch master is starting as can be seen in /database. It's not as simple as "reject a block that is not in the local transaction pool" because an initial download can never occur. Instead you reject blocks close in time to the transactions in your transaction pool if a mismatch is found.
On a conceptual level this is a very promising approach. If it works, it's not impossible
that it could become a new standard that replaces the old Bitcoin-based infrastructure
in cryptocoins.
Also, in this way if for some reason a node doesn't have a transaction it will still accept the block just prior to the next block due to the timing I've described. This can occur if your node came online and is not fully synchronized with the global transaction pool when it receives a block.
I'm not sure if I grasped that. Does a node accept a block with a transaction that
it does not know about?
Also, Vanilla has a Peercoin-style checkpointing. Will that become superfluous
once the 0-confirmation goes live?
