[Thanks, Raymond and Jamie]
Update: We're aware that the screen shot here shows a UK build -- we're currently awaiting official comment from T-Mobile USA on how the rollout is progressing over here. Stay tuned!
Update 2: So far, it seems like the only folks getting updates are those who'd previously installed a UK build on their US-spec G1, which means the good people playing by the rules and avoiding any firmware tomfoolery are still update-less. We're still waiting on official comment from T-Mobile, but it's not looking good.
Update 3: Indeed, the push hasn't started. T-Mobile tells us that "nothing has changed... plans are still to roll it out this week" -- so if you're crazy anxious to get this installed, your best bet is probably still to install the ADP or UK build.