what's the difference between Alpha and Beta codes? don't they give you the same content? I know for a while he said that Beta won't get you the game once it launches, but http://www.minecraft.net/prepurchase.jsp
seems to say that it shouldn't matter anymore.
When the game was in Alpha, the ToS stated that if you made a purchase you would get all Minecraft content, present and future, including any expansions that might be released. By the time Beta came around, Notch had lawyers, and they made him take that part out.
You'll still get the full game with a Beta code, just not any extra content. To be completely honest I think development will stop the day that the full game is released, Notch seems really bored with Minecraft, so it doesn't look like those who bought the game in Alpha (myself included) will get anything extra anyway.