They go by model up to a point. I know in some cases I've seen people all with the same model get it, and no other models get it until days or weeks later. Since we all know that not all models are created equal, it may be that they expect more problems with some than others and might try testing the waters with the 'good' ones to see if there are any problems that dont show up until lots of people get on it, and then go to the 'bad' ones later in case they need to iron out any wrinkles.
After that, who knows. I've seen lots of people with 2-3 receivers of the same model have one upgrade one week and the other two not get it for a week or more afterwards. Maybe they go by activation date or serial number or some other obscure measure.
After that, who knows. I've seen lots of people with 2-3 receivers of the same model have one upgrade one week and the other two not get it for a week or more afterwards. Maybe they go by activation date or serial number or some other obscure measure.