Just my poorly articulated thoughts.

We cann't go to Mr. Webster for an answer as to what is and what isn't cheating. A person can cheat on their spouse with out ever taking a step away from the keyboard. They can also have a physical relationship with someone other than the person they're maired to without it being cheating.

Any time one person in a relationship engages in a relationship with a third person (beyond the context of friendship) without letting their partner know about it it's cheating.

Emotionaly cheating has been pretty well discused and I agree that so long as your partner knows about it it's not cheating but there hasn't been alot of discusion about physicaly cheating on ones partner. If both parties agree that they want to bring a third person into the relationship then I don't consider it to be cheating but if either one of the people in the relationship isn't comftorable with it than it's cheating.

As for my personal experiance... I've had two relavent online relationships that apply to this topic. In the first it was just a friendship. The guy involved was someone I met in the chatroom back in the days of Voodoo. He was married and his wife didn't know about the site. As we started talking and getting to know eachother he started feeling guilty and like he was hidding something from his wife and so he stopped comming around.
The other relationship was with a guy who was still married to his wife but they weren't living together. He actually gave her my screenname and she and I still chat everyonce in awhile. He and I don't talk anymore though. We didn't have a falling out or anything. I think it got weird for him and I lost interest in him.