Nortel reports that -
OK...

I have rerun all of the tests with my traffic generator on.. not just
sitting idle.  The collector ran in all cases as expected.  Yesterday,
it was just sitting relatively idle... I will re-run that test now, but
it takes longer (obviously) to fill the heap.  Again, I cannot see what
the difference would be, but something changed here and I would like to
figure out what it is...

One thing I did notice... 

the ConcurrentMarkSweep MBean is not updated with the concurrent
collector runs... it only appears to get incremented if you kick a full
GC.  Not sure if you have a bug report on this or not, but that behavior
is consistent.