I don't understand some of these responses. At LHR T3, AA does not check the baggage size -- they are checked as you enter security (either Fast Track or regular), and my experience is that it is completely random, and you can get nailed in either line. If the bag doesn't fit in the sizer completely, they are pretty unforgiving, and unaware of your class of service when they are making the call.

I *have* found that if you don't stack your carry ons, i.e. if you have a laptop bag, carry it over your shoulder, rather than putting it on top of the carry on, for some reason the carry on looks smaller and you are less likely to be stopped.