Most games would be happy to sell three million copies in their lifetime. For Minecraft to have accomplished such a feat before it's even been properly released is one hell of an achievement.
Considering how much games are tinkered with and improved over time, Minecraft -has- been a released game for quite some time. It's like an MMO that starts out limited and a little buggy. It gets worked on for years and years to become something far more.
In all seriousness, until Notch comes up with something else, I don't think Minecraft will -ever- be done.