I heard Stephen talking about this on one of the steampunk panels at Eastercon.
As he described the world, the map would be upside down as the magnetic poles had shifted, as well as the continents being rewritten.
He also mentioned as a part of the hidden backstory that the world had been 'swelled' to about five times its current size by injecting mass from other planets and hollowing part of the world out during an ancient period of geo-engineering to squeeze more real estate out of the world. As part of the same project, extra planets and moons had been imported into the solar system, hence the noted discrepencies between Sol and his universe.
He also mentioned having a nascent story-line which would involve Molly and the other friends at Tock House doing a 'Journey to the Centre of the Earth'-style adventure.
I can't wait to read that one!
It would all have to be accounted for in a map, tho, I guess.