I think, based on the overall 'feel' of Civ 5, and there being only one religion icon per city, it is safe to say that there will only be one religion in each city at any given time.
I would definitely like a bit more of a robust system, less for gameplay reasons and more for thematic ones (and therefore unlikely, for a commercial release). For instance, you could have other minor (visible only in city screen) religions present in a city besides the dominant (icon) religion, which don't actually exert their religious 'effects' from belief on that city, but which make it easier for foreign missionaries to change the dominant religion. It could also act as an interval step for missionaries, IE, you have to establish your religion as a minority in the city first before sending a second missionary to make it dominant, and inquisitors remove all minority religions and downgrade a dominant non-state religion to a minority.
But I seriously doubt they'd implement that. Very probably, in my eyes: One religion per city, period. Missionaries add your religion to an empty city, Inquisitors replace not-your-religion with your religion, religious spread happens randomly and only to empty cities. Civ 5 has, generally, been more about accessibility and streamlining than Civ 4, so having a religious spread system that is unusually complex compared to other features doesn't seem likely to me.
