Susan, once lettuce bolts (sends up the flower stalk) you'll find the leaves are also quite bitter. So yes, it is done for the season once it flowers. This usually happens once the weather gets too hot for the plant to make sweet, tender leaves.
Down here I really don't get very much weather that is cool enough to grow good lettuce so I interplant with other types of greens that do better in warm weather. Spinach, mustard greens and 'corn salad' which is really mache are all delicious. You can plant most of them again starting in August or September and harvest sometimes well after frost, too. My daughter had mizuna, a wonderful type of mustard green, well into December in Salt Lake City last year.
