Guess what? It started growing again in September. I picked a little of it then but didn't think it would last long, with the weather getting colder. It kept growing, though, and I kept picking it.
We have had below freezing temperatures at night for the last week or more and this is what the lettuce looked like yesterday.
I picked some and ate it on a sandwich. Glorious, isn't it? I don't know how long it will last, but I'm going to enjoy it to the very end and next spring, I'm going to plant it earlier, now that I know it will withstand freezing temperatures and below.
I bought the seed from Seeds of Change. They probably still have it, but I can't guarantee that. All I know is that I am happy with it (and with a late autumn salad!).