I have had a couple of different types of dreams about my online friends. Sometimes we are the messages we post on the board. In this dream I am NOT reading their messages on the board, I am with them and our printed words are the embodiment of us and the messageboard is the physical space where we meet. In the dream, this arrangement seems perfectly natural to me -- it's just who we are.
It makes sense to me that I would dream of them in this way because other than a few Christmas cards and the occasional e-mail, that is my entire experience with them. Now I am moving into the world of ICQ, so it will be interesting to see if that surfaces in dreams also.
I have also "seen" some of them in bodily form in dreams, even though I don't know what they look like. That seems to me to be similar to dreams I have had of people I know only over the phone.