I realize this is anecdotal, but I know several people who purchased multiple 360s because they got sick of sending it in, which makes me believe that 360's install base was almost certainly lower than 85 Million. Reminder that only the 5th revision (Jasper) finally solved the RRoD problem for good, and that was 3 years in, and that all previous versions had a failure rate of nearly 100% as time went on.
Is there anyone here who bought only one single 360?