The port has a 6-dot brass contact array inside the groove for the ‘bottom’ strap connector slot. Several sources have confirmed its existence and placement to me. It is very similar to the connector for the Lightning connector in iPhones, as that connector currently only uses 6 of its 8 available pins. Apple recently began opening up the Lightning port for use by third parties. A source says that this port is currently for diagnostic purposes only — but that there is nothing preventing it from being a connection port for future accessories.
The port was spotted by our forum members in a number of the new Apple Watch promo videos that Apple released today.
Matthew Panzarino, the original author of the report, confirmed today that the port still exists, but he remains uncertain if it will ship in the final version:
Confirmed the 'diagnostic port' is still in the ᴡᴀᴛᴄʜ band channel. Don't know if it will ship that way or not. — Matthew Panzarino (@panzer) March 9, 2015
The Apple Watch will begin shipping in April with pre-orders starting on April 10th.