After reading comments here I read more deeply into the mojang/notch vs open tools stuff, and I rephrase my "good on ya" to a "nice delivery/flamewar avoidance in yer announcement".
To clarify my original comment, I was referring to the open playground approach of Minecraft, but making an unfounded assumption that the open playground model extended to the code. I made that assumption because of second-hand knowledge that mods were making it into the game. Mea culpa.
No he said it will be open source when he feels it's not making any more money. You can find this (or atleast use to be able to find it) on minecraft.net
Edit: Source
>Once sales start dying and a minimum time has passed, I will release the game source code as some kind of open source.
http://www.minecraft.net/about.jsp
Presume that he was only saying that to string along the people that would not have been otherwise interested, like the burgeoning community of open-source hackers working on various Minecraft tools.
Not that I know of; he worked for King.com, which produces proprietary Flash stuff. I don't know if he's released the source to his one-off games, like the Infinite Mario Brothers, but it would not surprise me either way.