You've all heard the famous "Nokia ringtone" many times (hard to describe in text, it's 10 notes, often satirized on Trigger-Happy-TV) and even the polyphonic version.

I suggest that a symphony orchestra, around warmup time, should suddenly play this song with their full glory and set of instruments. This would be funny on its own, but could then be followed by a very memorable, "please remember to turn off your cell phone now in preparation for the performance." It might actually get people to do it.