I think that when people are broadcasting (talking very loudly in a public place) as opposed to having a private conversation then it is considered OK to overhear a conversation. Sometimes when working whilst traveling on public transport I wish that I hadn't overheard.
Hearing a conversation in a physical public forum such as a conference isn't rude. It is designed to be a social, collaborative exchange of ideas and not the place for private discussion (of course this may be different if you have taken yourself to a quiet corner).
Given all this then it is reasonable to expect that peoples' behaviour must be of the highest standard as any slippages may prove costly (as mentioned above). I will still make bad jokes that hopefully will not offend anything but peoples' taste.
-- Stop your grinnin' and drop your linen...they're everywhere!!!