Avatars, alias and being what or who you aren't

I have been considering for some time now the long term implications behind the idea that we explain to children that they should not, online, in any way reveal who they are or where they are from. It occurs to me that this is an extension of role play into a real/virtual world situation. As I think of it, the idea seems okay for early years explorations but I am getting increasingly insecure about it as children get older. Should we be encouraging them to pretend to be someone they are not and not accept (by default) the implications of their real and virtual actions? It is a good argument to say that they are modeling in a true sense in situations that they can manipulate to see outcomes and can, for the most part, retract and come out unscathed. But is it socially viable, in the long term, as the blend between the physical world and the virtual one becomes blurred? ( Have you visited Second Life recently?)It seems to me that as we build the various levels of sub-worlds which people (children) inhabit then we are in danger of them becoming more of a reality than reality. Although I am not a 'film' person, can't sit long enough to get the whole thing, the film 'Matrix' had a profound effect on me when I first saw it. And the concept of 'worlds within worlds' has always fascinated. 'But doesn't everyone do this?' I hear you all shout ... so why am I bothered?