FYI, I've had the G1 and the myTouch, and updates were totally random.
Husbands who purchased their phones weeks after their wives got them (or the other way around) would get the update, while the spouse was still waiting a couple of days later.
At least with T Mobile, and I'd guess with Verizon as well, there is NO rhythm or reason as to who gets it when... simply because some people in California have gotten it does NOT mean it's rolling out in California specifically... it's totally random.
I believe they also do it this way so that if they begin to get a LOT of calls to customer service, with problems, they can halt the update release, and see what's going wrong with it, or fix it before it goes out to hundreds of thousands of people.
Just a guess on the last comment, but I'm sure if they wanted to, they could certainly release all one million (or whatever) updates at the same time!