When I first heard of this, I my self being a developer found a loophole in which banned players COULD've been able to join, I poked/msg'd col on the topic and Devin/Kpwn have fixed it.
In 1.8 everything (Permissions, Bans, Ops) will now be going by UUID (Universally Unique IDentifier) which in this case, names become dynamic but your account doesnt. One loop hole in this is to get the UUID of the player you would have to have the player login and record the UUID, so permanently banned players can change their name then login. Kpwn says they've fixed this, so I'm sure they have.
To answer your question, No. Bans have now been based on a UUID, therefore LeafyGreenTea, with the UUID of "xx.xxxxxxxx" changes his name to Joe333 would still have the UUID of "xx.xxxxxxxx" So no, Leafy couldnt just change his name and login.