We went Aug 07 and we're returning Aug 08! We found that it was hot and humid, but as a PP mentioned, there are opportunities to cool off, so take advantage of them. We were lucky to have no rain last year too! Very uncommon!
We bought a water mister fan at the parks (we brought 2 from home but they were cheap and both broke in a day) This does help keep you cool!
We also go to whatever park has early EMH to beat the mid day heat and lines. You can get so much done in that first hour!
Any restaurant will give you free ice water (the size of a medium soda, even if you aren't buying anything else, counter service too) so we bring along drink mix ins to help it taste good. Also, get a dole hwip in MK...beats the heat anytime! Have fun planning!