Married, But Separated-Is It Okay To Date?

So, I'm playing around on the internet tonight and I stumble across an article of a girl asking for advice regarding dating a married man.  This man is her best friend and ex-boy friend whom she feels has some kind of feelings for her. For some reason, he and his wife live in two different states (the writer doesn't indicate that there are problems in the marriage or that there is a divorce in process). Of course most responses advised the girl to stay away. Yet there is one response that was a little different. Check it out:

"well it depends...why is he 400 miles away from his wife? are they separated? if they are separated then it should be obvious to anyone that it is ok..as long as they are going through the processes of divorce. otherwise I can see how it could be morally wrong."

Reading this question has sparked some questions in my mind about how people feel about dating married men. My question is "would you knowingly date a married, but separated man and why/why not?"  How about a man that is not married, but has a girlfriend? Is either situation better than the other?