I have Beta 1.6.6 and yesterday in my Minecraft world i saw a 4x4 hole in the ground... i went down there to check it out, it was about 10 deep and at the bottom i saw "him" he was there for about 5 seconds then he ran off down this 2 block high corridor, ( i had literally found out about him the night before) and was already pretty spooked by him so i exited Minecraft and went into MCEdit to have a look at his cave, there was about 6 rooms coming off from the corridor so he could've gone ether way... later i told my friend who had my world, but i was on day 24 and he was on day 118, he said that he had seen odd this in the area of the hole like random dirt pyramids and dirt towers that he didn't placed as he didn't even know about his hole and had never even gone to the area i was at, apart from the brief time to check out the random piles of dirt.

if that isn't enough proof for you i don't know what is, I'm going to go and look for him soon, ill try 2 get a pic.