In which case, I wonder whether he (notch) is going to have trouble with the online performance. I've tried online play briefly and it didn't seem bad, I guess it isn't really twitch based.
I've played online (a few weeks ago) and there did seem to be problems with the server due to periodic GC causing massive lag and disconnections every so often.
It could also be people laying minecart track. This, unfortunately, completely kills the server and makes it need to be restarted.
in my experience, tracks on smp hosed the clients, not the server.