I believe that many times being given a complementary upgrade is "hit and miss". On several ocassions I have checked in at LHR w/AA and have been given my pre-assigned coach seat. This is usually at least 45 minutes prior to departure as getting through security can sometimes be a nightmare. Three times during the last three months I have arrived at the boarding gate to find out that I have been upgraded (LHR-EWR or JFK). My assumption was that coach had been overbooked, but I was told by the person at the gate that they did not know why I was upgraded. Of course, I did not ask a lot of questions just in case there had been a mistake. Of course the only problem will be my expectation next month of a gratis upgrade from LHR-EWR. Can I hope for 4 out of 4?