This is all we know for sure, and likely all we'll know for awhile, via Clarence House (British spelling theirs):
"Her Royal Highness The Duchess of Cambridge has been admitted to St. Mary’s Hospital, Paddington, London in the early stages of labour. The Duchess travelled by car from Kensington Palace to the Lindo Wing at St. Mary’s Hospital with The Duke of Cambridge."
That's expected to be the only official word from the royal family until after the duchess gives birth and Buckingham Palace is ready to make its formal decree—something that will happen only between 8 a.m. and 10:30 p.m. local time (or between 3 a.m. and 5:30 p.m. eastern standard), as per an informal agreement with the news networks. At that point we can expect all the breathless media coverage to culminate in what will likely be a rather absurd low-speed helicopter chase. Here's how the New York Times broke things down last week:
The duchess’s doctors will sign a birth notice. The notice will be hand-carried to a car. The car will be driven to Buckingham Palace. Then the notice will be placed on an easel in the forecourt of the palace, informing the world of the baby’s birth and possibly his or her name.
All the while, a news helicopter belonging to the British broadcaster Sky News, whose pictures will be shared with every network, will be hovering overhead, almost as if covering a slow-speed car chase. But that’s assuming the helicopter, stationed south of the city, can get there in time. News executives expect to get five to 10 minutes notice, at most, before the car starts on its short journey. A Sky News spokeswoman, aware that the world’s news media are counting on its coverage, said she anticipates that the crew will have “enough time to get airborne.”
Of course, that won't stop the media masses who have been camped out at St. Mary's for weeks from doing whatever they can to scoop the official announcement. As the Washington Post explains, the best chance of that happening would probably be from some random tweet from within the medical compound. In the meantime, however, we're all free to speculate on the hard-hitting topic of whether the new baby will be "a feisty Leo or a sensitive Cancerian."