Electronic agreements are a valid form of contract. One good example of them are EULA (End User License Agreements) that often accompany software. A contract requires three things: an offer, acceptance, and consideration. The offer here are the terms set forth in that document. Acceptance is shown by using the forum (or continuing to). Consideration is a legal requirement basically meaning both parties need to get something out of the agreement. This forum is a service which arguably gives benefits both ways (traffic to site on the one hand, entertainment to the user on the other). By using this service, arguably this is a contract.

One issue I see is on the offer/acceptance. These new terms are being laid out after the initial registration for the site, so the contract has already been formed between site admin and user. I guess it might apply to future users, but then again, there might be problems with the way the "offer" is made here within the site, rather than on the registration page. Really, it would be better to post it up in the registration page so that you have to read it, or at least be given a fair chance to, before registering for the site. Even then, however, with all the users who've already registered, I'm not certain you can go back and retroactively modify the contract. I guess you might argue that the user loses only the continued enjoyment of the forum by choosing to no longer take part and that just like, say, a restaurant may choose to change its prices or other various aspects of its business, so too may a forum change its rules and not be forever locked in to the original terms given during the registration. This all rests on the premise that a forum is analogous to a business of that sort, which does tend to ignore the fact that there is a definite registration for a forum, while there is none for a restaurant.

Having said all that, I'm not a lawyer yet and certainly not an expert on contracts. The internet has made contract law very complicated and I suspect there may be more specialized rules for these kinds of arrangements, though I don't know what they are. Still, it doesn't hurt you to have something written in like this. It certainly may make someone think twice about trying to sue since they now have to triumph both in contract law as well as in tort before they can get money from you.