tomatoes continue to ripen once they've been picked, like many fruits. Bananas help speed up the ripening process in fruits which is why you usually keep them separate to other fruits.
Thats a lie actually.. It's not just frost, but lack of warmth, so if the temps are going to drop rapidly then yes, get them of the plant (if the plant is outdoor)
I have had more problems this year than any other due to the intense heat in my tunnel (often 120f). But on the plus side I have less than 10 green tomatoes at this stage and a freezer full of real tasty ripe ones.
If you have them outside I doubt you will get them to ripen. Bring them in and put them on a window sill. But I am sure you are aware that they wont taste just as good when ripened indoors. But they will still beat any supermarket toms.
All is not lost as they will go well in tomato relish or chutney, just add the green ones as well as some ripe ones. I made a batch last week and it was such a success that everyone who tasted it wanted a jar to take away... need to make some more this weekend .. only I dont have any green toms !