Normally, Minecraft logic requires some time in-game to execute, but last year (IIRC) someone was able to invent "instant" logic gates and transmission wires, by exploiting some unusual interaction between pistons and the block-update rules. They're quite bulky, but they do get the job done of achieving FTL communication and supertask computation ;).

With an optimizing compiler to take advantage of these, it might not be unrealistic to implement Minecraft in Minecraft. The real-time speed would only be limited by the real hardware, not by the Minecraft hardware.