Every one of us picked our Minecraft user name when we signed up on minecraft.net for the first time (and every time thereafter). For most of us, knowing that whatever name was chosen is stuck with us forever is just a part of the game. Still, what if you gave yourself a silly or inappropriate name? What if you're just tired of the one you have? Currently, this means getting another copy of the game - but maybe not for long.
http://twitter.com/jeb_/status/196224261923414016 that username changes might be a feature in the near future, something that Mollstam is looking into currently. While there is no set date - or even a hinted one - for the feature's arrival, it would seem that it's more a matter of "when" than "if" now.
- Name changes to be expected in the future, will it cost or not, who knows

-- Seems everyone doing Hunger Game games now http://www.minecraftforum.net/news/484-hunger-games-by-hunger-craft/
-- The Minecraft Wiki also got an overhaul and now looks like the forums, rather than the basic wiki background, which I preferred now it looks daft. http://www.minecraftwiki.net/wiki/Shadow_Mob