Minecraft enters the indev (in development) phase with the 0.31 update. Paid users are able to test the latest survival updates, but Notch warns they may be buggy:
This is not official in any way, but I’ve gotten several requests that people who paid should be able to play the latest version, even if it’s horribly buggy and crashes all the time. And I agree. They should be able to do that, at their own risk.