Hi there Billy and others.
Thanks for bringing this up. To be honest, there aren't really hard and fast rules on this (one of the reasons why we always refer to guidelines rather than 'rules') in the same way as there aren't black and white rules on what constitutes a PA: clearly 'you're being a bit of a silly sausage' = fine. 'Fuck off you mean old cowbag' = not fine, but there's a world of grey in between so we tend to take each post on a case by case basis.
With comments about celebrities, it's similar. We do think people have to be allowed to discuss those in the public eye and express an opinion on them but if they are just being needlessly nasty we'll delete posts. However, to answer your question, we don't count those as a 'personal attack' in exactly the same way unless the person in question happens to be on the boards, involved in the discussion and has been upfront about who they are. So you're correct, we don't see mean comments about celebrities in quite the same way as personal attacks but that definitely doesn't mean we think it's OK.
Even with folk in the public eye, who presumably are more used to criticism, positive or otherwise, we do draw a line and would delete for not being in the spirit of the site, or just being plain mean. It's one thing to say A.N. Other Singer sounds like a sackful of cats going through a mincer on her latest album, but quite another to say she's a rotten parent. Equally saying a politician comes across as weak or foolish etc might well be OK, but saying they have a face like a bag of spanners might not. Both depend on context and relevance, among other things. As with everything, we just expect people to be reasonably civil to each other and about others. If what you're posting might draw a sharp intake of breath if you said it out loud in the pub, it'll probably get deleted.
And of course any posts would have to be reported to us first. So to answer your point, BabyBubbles, we don't trawl any threads for posts that transgress guidelines, so as always, if you think a post is a bit dodgy do just hit the report button and we'll always be happy to take a look.
Hope that explains things a bit anyway. Apologies for the short essay. (Bet you're sorry you asked now!)