Hautala on the selection process:
I know some of you have questions about how Windows Phones are selected to receive the update. Put simply, it’s done totally at random, with absolutely no preference for carrier, model, or country. Random sampling is an important engineering technique to accurately measure quality and pinpoint issues early. In some cases, this can have interesting consequences: A family, for example, can have identical Windows Phones, yet only one phone might get an update notification.
The exception to all of this is the Samsung Omnia 1.4 devices on AT&T in the US, which is still in relative limbo between Microsoft, Samsung and AT&T. More news on that when we have confirmation on the upgrade path. For now, everyone else can keep checking.
More details at Microsoft's Windows Phone blog.