We were married on September 13, 2010, with 5:00 p.m. ceremony at the Wedding Pavilion, following by reception at the Whitehall Patio/Room. It was unbearably hot and humid! In hindsight, I feel a little bit bad that our guests sat in that heat for the pre-reception. Some of them returned to the patio around 9:00 p.m. for dancing and they were soaked in sweat (but they didn't seem to mind).
Some people tolerate heat and humidity better than others, but you may want to keep it in mind if you have elderly guests. We shuttled my grandparents from ceremony to reception in the limo, which helped a little bit.
My biggest worry about having a September wedding was the risk of hurricanes. I've been to WDW in August and nearly had my flight cancelled because of the severe weather. If you and your guests will be flying to Florida, keep this in mind. Good luck!