Stuart,
I was always of the mind that "rules" for ethics (and for that matter, etiquette, civility, manners, etc) were in place for when times get tough, not when things are hunky dory. I mean, who needs to be underhanded or stealing jobs when there are hundreds to choose from? Who needs etiquette when everyone is getting along?
I left Chicago as a courtesy to my former employers, although I don't think its unethical to open up down the street.
I always ask if a gca is in place and won't go where there is already a contract. In the rare case where a change is going to be made for reasons beyond my control, I have asked the Owner to dismiss the other architect, with reasons, etc. before I will talk to them.
Sometimes, I have sat too tight, hoping my skills and knowledge will show through. Of course, going out and shaking hands, marketing, etc. is really the way to get more work, over any other method, as well as being the most ethical way.
Not sure there is really any line to draw. Either it feels right in the gut, and you know it, or it isn't. All the major societies have drawn the line long ago as what constitutes fair play among your competitors. Of course, there were always borderline cases.
TD, while I wasn't there, I believe the "real reason" the archie in question got booted out was he was just too much of a pain in the can to deal with. They all sort of agreed he didn't need to be there.....