I think the three dots are for inductive charging. No worries about Apple claiming that this is a copy of the magnetic charging ports of its Macs, as this type of charging has been around forever. For decades I've had cordless home phones with this technology.
The idea of a nexus tablet co-launching with the Nexus is cool, but if you look closely at the vid cap on that G+ thread, the alleged tablet would be half the thickness of the phone. I don't think that's likely. I think it's just a cool background graphic to accentuate the curvature of the phone's screen. Why would a tablet screen be curved?