Your best bet is to pick the fruit green before a frost and then ripen them in the house before collecting the seed for processing. If they've started to turn paler green at the blossom end, they should ripen successfully in the house in a single layer on the counter. The riper they are, the better in terms of seed being mature and viable. Some of the expert posters on the Growing Tomatoes forum suggested it, and I've saved seed that germinated fine from fruits ripened in this way before.

I agree! That's what I've done, as well. So, just let the tomatoes stay on the vine until your first frost threatens.