We went late August/early September of 2007 and yes, it was hot and humid... but no more so than in Ohio where we live so it was no different than being at home.
Having said that, at home I am not out in the environment 12 hours a day either, I am truly out very little, ecspecially on a really humid day. While we don't have AC (we do have one window unit), our home is a lot more bearable than being outside.
At WDW we drank A LOT of bottled water, dressed cool and comfy, I bought a spray water bottle that holds ice (we shared), and we brought (ugh

gross) "sweat rags" to wipe your brow. Son and I both have asthma, thankfully a TS dinner allowed for quite a cool down period daily.
You will be hot, but fine, for the most part!

IF it gets to be to much, I would say go back to the resort and take a nap in your cool room, or a dip in the pool!

Also, when it rains (not storms, no lightening...but simply rains) enjoy it!

The park clears out a lot, but a lot of attractions are still open/run... it is awesome!
