brotherhood_of_LAN - 4:53 pm on Nov 11, 2012 (gmt 0)
In short I think the answer is "extremely difficult but possible".
Aside from hiding your WHOIS & details from your host... you would likely want to use a proxy too.
Consider that someone in the forum may be able tp post an image to a thread & therefore get the IP address of everyone on that thread visiting the page. Even when using a proxy it is possible to begin to guess where someone lives based on the delay in roundtime of a request.
I guess it's a question of how much anonymity you'd like and how much you need... as we could count up the dozens of ways in which someone may be able to better identify you.