I have a radio where I can listen on a second frequency, so usually have it on Area and I have twice had ATC advise of "two aircaft in the vicinity of xxx at altitude yyy on a potential collision course" - one of which was me. I responded to one of the calls saying I was altering course and altitude, but ATC did not communicate further - I guess they were either busy, or just felt the initial advice was all that was necessary, especially as I was altering course. I also have a Mode S transponder, so they would have been able to see me.
I went with a bunch of other pilots to a talk given by ATC at Melbourne airport, and basically they said they do not want to hear from us in the normal course of events, although obviously in an emergency they would respond. I am pretty happy if they continue to just broadcast if they see two VFR aircraft in the same area and altitude on a potential collision course as obviously that adds to our situational awareness, and makes us look around rather more in an appropriate direction.
Neil