Via Bruce Schneier, here’s a New York Times article revealing that you may be monitored while you’re on hold:
It is at these times that monitors hear husbands arguing with their wives, mothers yelling at their children, and dog owners throwing fits at disobedient pets, all when they think no one is listening. Most times, the only way a customer can avoid being recorded is to hang up. “You could have a show on Broadway just playing the calls,” said Mike Schrider, president of J.Lodge, a call monitoring service based in Hammonton, N.J.
This isn’t news to me. While in university, I used to work at Tourism Victoria. We would sometimes put people on hold using the mute button on our headsets. They couldn’t hear us but we could hear them. I know this isn’t exactly the same technology, but the result is the same.
DarrenBarefoot.com 
Dear God! When did you work at TVic? I am also a survivor, having worked there 1990-91 and 1993-94.
It’s a small, small world
When I worked at a call centre, I’d do the same thing, muting my headset but still being able to hear the caller. Once I heard the caller say “hey, this girl sounds really cute” (I’m assuming to his friends nearby), which is proof that my professional phone voice was much, much different from my usual dry, sarcastic tone.
Stephen: Hmmm…maybe 93-94 or 94-95…I forget. Down at that sweatshop on the corner of the Inner Harbour, right?
I used to work at McDonald’s and people never seemed to realize that we could hear what they were saying when they were in the drive thru.
When I worked tech support, we’d often use the microphone mute rather than hold. The effect is similar to the caller, as all noise dies on the line.
People would make confessions and have knock-down fights in the 30 seconds I’m asking my neighbour for a second opinion… “I won’t tell him you dropped the laptop – what, do you think I’m stupid?” or “This tech guy is ignurnt” or any other sort of aside that doesn’t help their case.
At work here in our support department there is a British guy who is of Indian descent. One time he had a client on microphone mute while he asked me something and the client starts going going to his co-worker “Man, we got put through to their call centre in India”.