http://twitter.com/#!/notch/status/16791411340877824
http://notch.tumblr.com/post/2175441966/minecraft-beta-december-20-2010
We will also change the license to remove the line that promises all future versions of the game for free. Please note that this change only affects people who buy the game after December 20, so if you got the game for during alpha, you will still get all future updates for free, despite this change. A promise is a promise.
Buying the game during beta will include all updates up until release, of course, and bug patches will be free.
"Until release" being the thing to note here. But that tweet contradicts what he said in my quote, so I don't know what to think anymore.