Given that Minecraft is already a multi-million dollar property, it's perhaps not a huge surprise that Minecraft on the Xbox 360 sold very well in its opening 24 hours of availability. That said, Minecraft on the Xbox 360 didn't just sell very well. It sold so well, in fact, that it broke--nay, obliterated--Microsoft's previous XBLA first day sales record, and was profitable within an hour of being live.
While Microsoft is not the type to actually dish out real sales numbers--because lord knows what that information could do were it to wind up in the wrong hands (read: anyone else's hands)--but Minecraft creator Markus "Notch" Persson, being the delightfully free spirited wizard man that he is, doesn't really concern himself with these sorts of things, so he threw out a number anyway.
According to Notch, there are already more than 400,000 players active on the leaderboards. Now, that's not a 100% completely accurate number for overall sales, but it's a number that's been more or less (surprisingly) confirmed by Major Nelson.
Anyone want to take odds on how long it takes to hit a million in sales? Granted, we may never actually find out if it hits a million, since again, Microsoft is usually pretty stingy with that data. But I'd hazard to guess that, should it hit that milestone, they'll say something about it. I mean, it just sounds so impressive. "A million downloads." Rolls right off the tongue, doesn't it?