Ah, the circle of game development is a beautiful thing to see in action. Minecraft's name change feature has just reached the second stage of the time-worn process. First it was teased, and now it's been announced. Stage three, of course, is implementation—due to occur on 4 February. That'll be followed by the fourth and final stage: die-hard fans complaining that it's ruined the game.
Mojang's announcement reveals that name changes will be available from the Mojang account page. There are a number of rules governing how the process will work. Expectedly, players won't be able to claim existing names. It'll take 37 days after a name change for a player's old name to be made available for all, and players can only change their name once every 30 days.
Perhaps most importantly for server owners, banned players who change their names will still be banned under their new moniker. The name change function isn't a way for jerks to circumvent their bans, but rather a way for xXx_b0ngl0rd420_xXx to feel a little less ridiculous each time they log in.
For full details, head over to the Mojang FAQ.