It's undoubtedly a colossal success for a game which has yet to be released--it entered beta only last month--and was initially worked on by one single person, creator Markus 'Notch' Persson. As the half-million mark was hit in late October 2010, the dig 'em up sold that other 500,000 in a touch over two months.
While the alpha version cost only €9.95 (around $13) and guaranteed all future Minecraft updates for free, now you'll have to buy the beta version, which is priced at €14.95 ($20-ish) and only promises bug fixes for free post-launch. When the game does finally launch--expected to be "sometime late this year"--the price will go up again.
Some of the profits from Minecraft have gone towards building up Mojang Productions, Notch's studio, which continues to work on Minecraft as well as a secret new game.