When is it ethical to perform at another magician's show? I have gone to several friends and aquaintances (sic) shows, and I refrain from performing as they are being paid to perform. Then again, I have been to several gatherings where several of us were doing magic (but no one was being paid by the venue) and felt no onus whatsoever in performing. When is it unethical to perform at someone else's show?

If the person was requested to perform at the venue,paid or not, it is not a good idea to perform at the venue. This includes formal shows, if they are doing walk-around or table hopping, or even if they are requested to "show a few tricks". They are the one being requested to perform not you, and it may seem you are trying to show them up or steal their gig. Let them have the limelight and set back and enjoy.
It can be annoying to be ask to perform and someone else is doing a "look what I can do"

Craig

Agreed, Craig.

In fact, some of the magic societies have bylaws that state when you're watching another magician, you agree to be the perfect audience member- supportive and attentive (especially if you happen to get called up on stage.)

The only time I'd perform at another magician's venue is if that magician specifically invited me, as a fellow magician, to do a trick or two. Otherwise I'm just an audience member like everybody else.

I agree, I think the only time it is acceptable is if you are specifically invited. Many magicians would say that even performing on the street outside of a magician's venue is disrespectful!