Mojang just dropped one of the most interesting hints they have shared in weeks. Today Jens, the top Minecraft PC developer, shared on Twitter that the ability to change one's user name is coming in the near future. He posted on Twitter earlier today, "One of many FAQs: Name changing in the PC version *will* be supported, but not before 1.8 and an update of our web services."
Back in 2011, Mojang announced a new way to connect all your games purchased from Mojang called 'Mojang Account'. Mojang Accounts allow players to manage all their Mojang games in one place, like this:
Mojang has made it easy to transition from a normal Minecraft account to a Mojang account with their migration tool. Mojang Accounts also have numerous advantage over normal Minecraft, they are more secure than old Minecraft accounts and support adding multiple Mojang games to your account (but not multiple Minecraft accounts yet).
If you aren't sure if you have a Mojang Account or not, it's very easy to find out. If you have to log into Minecraft using your email address, you already have a Mojang account. If you log into Minecraft using your Minecraft username, you should transition to a Mojang account by following the steps here.
UPDATE: The main system admin at Mojang, Kristoffer, has cleared up some unanswered questions about name-changing. He said on Twitter, "Regarding Minecraft name changing. You will of course not be able to change to a name that's already taken by someone. Just to clarify" He also confirmed that changing your user name will be free, and that after changing your username your old one will eventually be re-released into the 'pool' of available names. In addition, Nathan Adams confirmed that whitelists/banlists will automatically switch to a new format so you can't get around them by changing your name.
Source: Twitter