Nothing but crickets chirping.
I think it will go like in this timeline:
1. gossip and speculation (that's the point we are in now )
2. THE ANNOUNCEMENT! much rejoicing.
3. two screenshots released, analyzed in great detail for two weeks straight (that green blob? it must be a new resource, or a Polish lancer)
4. more info and screenshots, Arioch's Civ Analyst site makes again good effort with keeping things up to date
5. final info for new civs, units etc. released (threads explode in rage as still no Kingdom of Sardinia civilization!)
6. the expansion arrives! everything is beautiful, nothing hurts.
Firaxis doesn't handle the Mac ports.
They're working on something. Whether it's an expansion, DLC, a patch, or VI is unknown, but you don't let a profitable game series collect dust and you don't have a development team sit on their hands.
The fact that they aren't talking and haven't put anything out in months indicates that whatever it may be is big.
The Steam database leak may be incidental. It could've just been someone at Valve screwing around. But the silence from Firaxis speaks to something big.
The longer they wait to announce it, the likely it is to be bigger.
When is PAX East?