I think the risks with starting a relationship over a forum are the same risks with any long-distance online relationship. Not only is there a physical distance to hurdle, but also a problem with the fact that the person on the other end of the screen is only giving you bits and pieces, then you have to fill in the gaps. Not a very thorough way to get to know someone.
I'm not saying it can't work, but I'd try to bring the relationship out of the online sphere before you get too attached.
How far away is the person?
“Inside every cynical person, there is a disappointed idealist”--George Carlin