In today's NY Times Magazine the following question was posed to the the Ehicist Randy Cohen:
"When we visit museums or historical sites my wife eavesdrops on the narrations of professional guides, typically two or three times for three or four minutes during a paid tour. I say this is freeloading; she says it’s incidental overhearing. What do you think?"
To Read his response, head here (it's the last question):
http://www.nytimes.com/2009/11/08/magazine/08FOB-ethicist-t.html?ref=magazine
For the record I do admit to doing this and Rick kind of encourages this in some of his guide books...so I guess I'm unethical:)