There have been a number of cases where legal action has, quite correctly, been taken by a number of people who have been subject to vile comments by others. I Have never used or would use Twitter but I would be curious to know how people who have been sent vile messages have been able to identify who has sent them ? Clearly where legal action is taken then the courts would be able to make such sites to release the necessary details but some people who have been on the wrong side of these comments have been able to get this information without going to court and use it to expose these people in the press, so how do they manage to get this information ?
I was watching Newsnight cover this + Paul Mason said something about a user being traced via Twitter & Facebook (using the same user name) - how they did it, well it was not publicised for obvious reasons of security. Maybe have a look on iPlayer at last night's program may give you your answers (or get you closer?).