Minecraft is still in alpha, but is rapidly nearing beta. I've only got one more feature to add; multiplayer support.
I'm renaming the old version of the game "Minecraft Classic", and will be slowly phasing it out.
I won't remove it, though. Just hide it.
The main branch of the game is currently called "Minecraft Alpha", and is under heavy development. It can be a bit unstable at times, so don't get too attached to your save files yet.
You can read more on the development blog, or here.