Taken from my 3L outline, the surrent obscenity test:

Obscenity – Miller Test (Miller v. California)
Whether the work, taken as a whole, appeals to the prurient interests;
Graphically portrays sex in a patently offensive manner;
Whether the work, taken as a whole, lacks serious literary, artistic, political or scientific value - in later cases, this was modified to "wholly without redeeming social value."

From wikipedia: "The first two prongs of the Miller test are held to the standards of the community, and the last prong is held to a reasonable person standard. The reasonable person standard of the last prong acts as a check on the community standard of the first two prongs, allowing protection for works that in a certain community might be considered obscene but on a national level might have redeeming value."

This doesn't help much. The problem is, there is no one thing that makes a porn "obscene." As Justice Potter Stewart put it, "I know it when I see it." The current rulings use the this test to determine if the work is obscene so using the Miller Test will be less taxing than reading through a bunch of recent cases which will just refer back to Miller over and over again.

I'm not sure this it topical for this group, however...

One of the open questions legally is how much a carrier is responsible. The right answer for open minded, free information folks is that the carrier should not be responsible. If the carrier is held responsible, then it becomes comcast's business to keep you from seeing or looking at whatever they think might be suspicious. Not offensive, necessarily, but anything they may even find suspicious, which puts them squarely in the position of needing to censor your access to the internet. And we really don't want that at all.

Another open question is who qualifies as a carrier vs who qualifies as a content provider. Tribe could arguably be either one. To a degree, they host content on their servers and that is the point of tribe. OTOH, they dont' really provide any content of their own, but rather, provide a forum in which their members provide content for themselves. So in a way, they are more like a carrier than like a content provider.

The board where a specific moderator is in charge is a less interesting question. If there's a moderator, they're responsible for content. A more interesting question are situations like usenet where there is no moderator, no specific location for the data, and all participants are carriers not content providers.

Within the moderated board category, there are still open questions. All tribes are moderated, (that's the official answer, although some technically are not). All livejournal communities are moderated as well, although there's really no way to hold those moderators responsible for content. If there's a content complain, typically the provider will investigate and may close the particular community or tribe, perhaps even ejecting the moderators.

If you maintain your own site, you're responsible. But in some cases they've elected to go after service providers like your ISP, your IP provider, your dns provider, etc. This is why it's currently difficult to find services that will allow adult sites and why adult sites current cost considerably more to host, even aside from the traffic involved.

My opinion, (I'm not a lawyer), is that you can't have an "unmoderated" forum if the forum itself has a clear owner. To have a truly unmoderated forum, you also need a technology which blurs the question of ownership, responsibility, and location. Unfortunately, as we've seen from usenet, this also makes it extremely vulnerable to being inundated with spam. And the best way we know to control spam right now is through moderation.

Oh, and there's a "best effort" question. If your hypothetical nasty porn was posted and allowed to stand is a completely different situation from having it posted, only to be removed by the moderator as soon as he noticed it.