And I was also leery of that time of year. But I posted some queries, here and elsewhere, and the advice boils down to this:
Go early. Take advantage of early morning when possible. The crowds will be lighter, and the weather (relatively) cooler. Temps will be in the 80s instead of the 90s.
Leave early. You should plan to be leaving the park of choice by noon. Earlier, if possible. The heat will become intense -- and so will the lines and crowds.
Plan to spend the afternoon poolside. Splash around, or go someplace with lots of a.c. Expect a 20-30 minute torrential thunderstorm every afternoon, too. The good news: often, the waterparks will clear out during a rain storm, so you might try dropping by Blizzard Beach or Typhoon Lagoon after a good soaking.
Rest up and then head back to the parks in the evening. It'll be cooler (relatively) and there's just more STUFF going on -- later hours, added fireworks and parades and the like.
If you've been to Disney a time or two before, and don't HAVE to squeeze in EVERYTHING, so much the better. You can pick and choose what you want to see, prioritize, and actually enjoy yourself. Some have told me they've found the heat at Disney World no better or worse than the heat back home -- and anyway, where would you RATHER be? Sweating in your own backyard, or Mickey's??
I'm looking forward to my trip. How about you?