I don't think I'd tell on them, but I would feel damn uncomfortable, since they were sneaking around, and I'd be the only one knowing. Since this is my second favorite Buffy forum, I might just leave and stick to my favorite forum. Not sure what I'd do if I found out they were there too...
Everytime someone whined about that person on the board, I would feel uncomfortable. I would so have to watch what I was saying a lot more (not necassarily a bad thing, because it would force me to be more constructive, but every once in a while, I need a good negative rant).
I think I'd be more okay with it if they were "public" members. We've had "celebrities" on another board, and that worked fine, was fun even. I think it made us me more condstructive as well, knowing that a writer or artist could read what we were saying about their work. But if they were hiding it, and I was the only one knowing, I'd feel weird.