I assume that the phone goes and checks occasionally to see if there's an update available. I also assume that the source they check determines if an upgrade is available based on some UID.

I don't know who controls the source, though. I would assume Google, but I could be wrong. I also don't know their algorithm for the order of the phones. It's possible that it's random. It's possible that they just allow the first x phones every y minutes, or some other timed first-come-first-served method. It's possible that they're picking specific UIDs. Since T-Mobile doesn't know my UID, and I didn't get it first, I kinda assumed I'd be last, or at least random. I waited a day or so, checking occasionally, and it still didn't offer it. So I just bit the bullet and did it manually.

I'm sure the reason they stage it is to not kill T-Mobile's network with all the phones getting the update all at once. (Though it's like 11MB, so it's not like a huge amount of data.) As such, they should check the source, and if it's not a T-Mobile IP, allow the upgrade. What do they care if it kills AT&T's network?