I love answering the questions of new parents, as DH and I are new parents ourselves. Our DD just turned 10 months, and has been walking since last month. We were just at WDW for Mother's Day weekend, and she had an absolutely wonderful time. We all had a great time!
DH and I went B.T. (before Trinity, our daughter) in 2001, compared to our most recent trip with Trinity it was a lot different. We were less concerned about riding all the rides and seeing all the shows. We wanted to relax and take things slow, just take in the magic of Disney with our daughter. And we loved every minute of it, we really had time to stop and smell the roses.
We did the baby swap a few times. How it works depends on the ride. For some of the rides, one parent stays w/ the child outside the line and the other parent rides. When the first parent returns, then the other parent can go through the fast pass line w/o waiting for a designated time like normally. For some other rides, you can both get in line and then do the baby swap in a designated area near the loading and unloading passenger area.
I would suggest to just take things easy, and go with the mindset that you are going to be on baby's schedule, not your own. You won't have to worry too much about the heat when you go in Sept. We stopped often to give DD water, and rest in the shade. If your wife is nursing, she can do that in the baby care center which they have at every park. DH and I made a several trips to the centers to feed and change Trinity. Also great place to relax b/c it's airconditioned and away from the crowds.
You will hear a lot of "isn't the baby too little?" or "the baby won't even remember it, why go?". But if you and your wife love Disney then don't let such comments discourage you. The memories you have will be worth it, and of course you'll have lots of pictures and stories to tell your little one when he or she gets older. DH and I had a wonderful time and it was definitely an experience we'll never forget. Seeing WDW through the eyes of a child is like going to Disney for your first time. Absolutely magical!
Please feel free to email me or PM me if you and your wife have any more questions, like what to bring, baby carriers, or any other tips. I love being helpful!