I don't think it's ever simulcast in that sense. We've had BBC America show episodes on the same day the BBC debuted them, but not at the exact same time, due to the time-zone difference. Not only is the US several hours later, but Doctor Who airs here as a prime-time show, whereas I think it's late afternoon/early evening in the UK.
Now this time as with the annoucement of Peter Capaldi as the next Doctor it supposed to be simulcast at the same time, so it'll be on at 3PM EST here.
I believe I remember the Capaldi announcement having commercials. So, while it may start at the same time, about 20 minutes in, I believe BBCA will fall behind when they start showing commercials (Once it's started, there's probably much less danger of people being spoiled, because they'll be watching the show, not looking for spoilers on what's happening 20 minutes later, etc) Maybe that's incorrect, but, I'm pretty sure I remember commercial breaks on the Capldi announcement.