I've been using "DrKillPatient" or some minor variation of it (e.g. "Dr. KillPatient) since... 2009 or so? My first notable username was just some amalgamation of initials at the time. I do not remember my exact reasons for switching, but I can definitely guess: I have somewhat mild OCD, although enough to get irritated when people pronounce things incorrectly (interestingly, this also holds true for incorrect/unnecessary/redundant terminology, such as "raining cats and dogs", "it is what it is", and euphemistic language in general), and my username was likely not easily pronounceable. I switched names shortly after I started playing games that use microphone communication-- TF2, I think (oddly, the Medic is/was my least-played class, despite my name)-- so I suppose that's the reason, anyway. A good friend of mine had the name of "Dr. KillPatient" when we played around with multiplayer in RigidChips, a sort of vehicle simulation game, several years ago. He chose a different username when we moved on to other games, and I took his, for reasons unknown to me.
I can't think of anything that I frequent in which I don't use my current name. Youtube, maybe, but even then I haven't used that account much since the Google merge. As for the permanence of this name, my "online" personality isn't vastly different from my "real" one (parenthesis-laden rambling monologues and all!), as is the case with some people. It's a bit more exaggerated, perhaps. Regardless, I don't see any reason to change it any time soon, as I think that I would only do so if my persona changed significantly.
On another note, I don't see usernames as too important, except maybe as a first impression, because after a while you get to like/dislike the personality behind the name without all the extra barriers that are found in the "real" world: appearance, race, economic class, religion or lack thereof, personal interests, etc. (All things that, I think, can cause biases even in people who don't consciously intend to have them.) That's a very interesting and valuable feature of the internet for me. I might dislike all but one of someone's interests, and yet we can potentially have a pleasant discussion on that one shared preference without anything else getting in the way, because it's up to each of us to choose what we want to reveal or say about ourselves. That's not something that I could see happening so easily anywhere else.