I have confirmed this bug. I have a save game with 69 cities. One of my gunships is poised to take over Siam's last city. When you do so, it appears to work fine. However, proceeding to the next turn will then crash the game.
Also, proceeding to 71 cities within the same turn crashes the game immediately. This is reproduced by conquering Beijing after Siam's city - easy to use the Stealth Bomber just to the east of Beijing to reduce its HP then advancing with one of the Mech Inf to the north.
I'm always connected to Steam, so I have 1.0.0.20 I suppose.
Save here:
http://www.yourfilelink.com/get.php?fid=576894
Also, as Vader1025 said, saving the game when you have 70 cities corrupts that save, making the game crash if you try to load it.
For now, my only recourse is to give back 4 cities to a couple civilizations, then steamroll the remaining 3 capitals.
My system:
Intel Core i7 950, HT on
GeForce GTX 295
6 GB RAM
Win 7 64 bit
pretty blue lights on the inside
My thought on this is: because they changed the win condition of Conquest to elimination of the capitals rather than taking over 75% of the land/annihilating everyone, they likely never playtested it out to 70 cities. Which is weird though, because among the beta testers, weren't there some that liked to do go out just conquer a ton of land? I guess not. Personally, I didn't realize the win condition was conquer the capitals until just a couple turns ago, assuming that these conquest conditions were similar to Civ 4.