Glad people are reading and discussing this issue.
As for guidelines for guys, every woman is different. So the "line" to cross will always be in a different place. Rule of thumb is to err on the side of caution.
For me that means talking to the person in the main Chat first, then if they seem responsive to you or if you have a private question, using the PM. Even then, don't just go straight for sex talk. Have a conversation. You know, hi, how are you, where are you from, what brings you here tonight, what do you like to see, are you looking at any pics right now...etc. Let it be a natural progression.
Sure, they may be in there looking to get a little turned on, but you don't know what will do that. Also she may be already in PMs with others or just not interested in chatting that way.
Funny thing is this kind of etiquette goes for anyone in Chat. I'm a guy, and I get annoyed when someone I don't know just pops in and PMs me wanting to swap pics, trade IM addresses, or asks me if I want to jerk over forum pics at the same time or something.
Basically just treat people as though Chat is a real life Room you just walked into. Even if we're all here because of facials, half the time the Chat isn't even about that or even anything sexual. It's just the nature of the beast called conversation - it goes off-topic.
Imagine if you burst into a room with your dick out yelling "which bitches want a big fucking cumload on their face?" And then you realize everyone is fully clothed and drinking wine while talking about music, totally ignoring you or rolling their eyes at your entrance. FAIL!
So just be cool and don't push anything, and you should be fine.