Even more impressively, this appears to be for the new protocol version - the new protocol is significantly harder to implement a server for than the old one (I say this having written custom servers for both in Python), and requires, among other things, a full lighting implementation in the server.

I'd be curious to see how this compares to the Java one for speed (and some other SMP servers that are springing up) - it's quite difficult to store an infinite size map efficiently in memory and still keep good random access times.