Hard to tell for sure.
In some games and with some civs, it was just like in Warlords... three, four, five cultural switches (once three + four, as it was happening on two borders).
In other games, it seems to happen more rarely and I've had games were I felt culturally dominant and grabbed land, but it never went to city flipping.
I'm not sure it's all the game and not in part my game style. My style changed with BTS because cultural victories have become almost passive at the level I'm stuck at. I use the cultural slider mostly for happiness now, I don't build a few wonders in future legendary cities just for the culture anymore, and I've slowed down a great deal on religious buildings and theaters - and that might be why I see less cities switching I guess this might change once I get used to the new military tactics of the AI and get my defense act together enough to move back to my old playing level.
However, the AI - some of the leaders at least - the same who attempt cultural victories, are now far more adept at keeping their culture afloat in their whole empire and to see you as a cultural threat to their border cities (they will build monuments, theaters, lib & universities and spread multiple religions for the extra temples and to get their slew of cathedrals etc.), and I suspect that if you want those civs's border cities to switch culturally, you have to work for it - move the slider up, have one cathedral on the border cities etc. With other civs, it seems as easy as ever or more. I had a game where Tokugawa lost all his five border cities to me in the course of the full game (space race victory, in the late 1900s) and I had reverted to an old strategy and built some cultural buildings (the basics, no extras) there just to ensure ressources would not fall to a cultural contest between us. But even without cathedrals and multiple temples I seem to have overdone it or grossly overestimated him as a cultural threat
