That's why I'll only call traffic to ATC if I'm reasonably
sure I'll *keep* site of the traffic.

Same here... as soon as I catch sight of the traffic, and call it out
to ATC, it just becomes part of my regular scan between instruments and
outside. I also try to remember to continue to scan for other
unexpected traffic, but definitely keep checking back at least every
few seconds so I don't lose sight of the plane.

I'm relieved that the consensus seems to be that calling out traffic
eliminates the IFR separation, because that's the way I've thought it
works for a while now (I think my instructor explicitly told me so a
couple of years ago tho). Case in point as to why this is a good thing:
I was flying for a night checkout at a flying club I recently joined,
and while transiting near a Class D airport (Westchester County, NY),
ATC called out traffic 9 o'clock descending from 4000 to 3000. I was at
2500, VFR flight following. Since I was in the way, ATC couldn't clear
the guy down for approach into Westchester, but I knew that as soon as
I called the traffic out that he'd be able to clear him down. I
couldn't get a word in edgewise, and by the time I could, he was
already passing overhead, so my call was "... traffic in sight no
factor", which was immediately followed by ATC call to the other plane
to descend pilot's discretion.

I think this is an important thing for pilots to understand, since some
may operate under the assumption that separation services will still be
provided even if they call out "traffic in sight". (Which they might be
- unless ATC says "maintain visual separation").

--
Guy