I just got back from WDW yesterday with my 4 month old daughter. She did great and we saw so many other babies about the same age who were doing really well too.
Before the trip, I bought a $17 Graco Umbroller (I guess it is their model that is in between an umbrella stroller and a regular stroller) that was big enough for a 4 month old, but really easy to fold up and get on and off transportation. If you don't have a really light weight stroller you might want to invest in a relatively cheap one for the trip. The Umbroller had a decent shade for DD, a cup holder for Mommy and Daddy, a pretty good basket, and it reclined well.
DD went on many rides, but she seemed to enjoy the shows the best, especially Lion King at Animal Kingdom and the Little Mermaid at MGM. Here's a list of the rides that she went on:
Pirates of the Carribean
Peter Pan
Dumbo
Snow White's Scary Adventure
Tiki Birds (she loved that one!)
Universe of Energy
Honey I Shrunk the Audience
the Figment ride at Imagination
The Great Movie Ride
Who Wants to Be a Millionaire (she loved the lights)
Muppets 3D
The Little Mermaid
Backlot Tour
Safari Ride at Animal Kingdom
Lion King
If you have a Baby Bjorn, bring it! We put DD in it for many of the ride just for a little added security. The only show that I took her out of was the Stunt show at MGM. It was way too loud for her.
I nursed my daughter on the trip, but I always brought an emergency bottle just in case she was alone with grandma and grandpa and they needed it. I just brought along a bottle or two filled with nursery water and powdered formula in one of those containers that has three sections. My daughter doesn't have a problem drinking room temperature formula.
The Baby Care Centers at Magic Kingdom and Epcot always seemed to be convenient for me when I needed them. MGM was probably the least convenient and the most disapointing. Animal Kingdom's was the nicest. I had intended to just nurse my daughter whenever and where ever, but she was way to distracted to have any part of that. The Baby Care Center offered a quiet and cool retreat when she needed to eat.
Good luck! I'm sure that you and your family will have a wonderful time. I can't think of a more infant friendly vacation destination than Disney!