from Emmerdale - a bloke leaves in a strop, his girl-friend says to her friend "He'll come round soon enough. Back row at the cinema tonight. [Snog's on]" The last bit in the brackets sounded like it. Does it make sense, meaning "there'll be kisses and all" or just a case of me mishearing something?
Ha ha! I don't think you misheard! Yes, we use ".....'s on" to mean that something is going to happen or has been arranged. For example, you're thinking of having some friends to your house for dinner, you spend all morning contacting them to see if they're free and if they would like to come, and finally everyone says yes. You would say to them, or perhaps to your partner who will be in the house that evening, "OK, dinner's on!" You mean that it's definitely going to happen.
In your example, I imagine the girl meant that she knew her boyfriend would be back as they were going to the cinema that night, would be sitting in the back row and therefore kissing would definitely happen!
