We went to DisneyWorld last year when DS2 was 6 months old. We had a fabulous time. All around the park there are Baby Care centers, which were great for getting out of the heat (lots of A/C), had changing tables and rocking chairs for nursing/getting babies to nap. There were also water coolers, and they had baby items for sale in case you forget anything.
I'd suggest that you keep your expecations low, because you never know how the baby will do with the heat/crowds/noise, but Disney is *meant* for families and there is a lot to do. Bring a sling AND a stroller, so that you can stroll the baby around the park but keep the baby in your arms if you're in line. Also, you can always take the baby on boat rides, the calmer train rides, etc. And if you and DH want to go on more adventurous rides, they have a great Baby Swap program where one of you can wait at the beginning of the line then "swap" the baby so the other can go on the ride without waiting in line again.
Have fun!
******
Erin
DS1 2/04
DS2 5/06
DD 10/09