From my website:
Walt Disney World in August
You may be crazy, but at least you won't be alone
Troll the various WDW message boards and newsgroups and you'll get the impression that no one who knows what they're doing goes to WDW in August. Maybe that's true, but up until last year, that's when we would always go.
You're going WHEN???
There are several good reasons to go to WDW in August besides the obvious one - it's the only time you can go:
Everything opens early, everything closes late
You want to be in the theme parks at opening, at closing, but not in the middle of the day.
It's HOT!
Everyone complains about the heat in WDW in August, but I can't tell you how many December, January, and February trip reports I've read on the internet where the writer complained that he or she was freezing. In summer, all of the pools and waterparks are open and very inviting. At many of the resorts the pools are open late, and it's common to see families splashing around and having a great time at 10:00pm.
The rain is our friend
You can use the daily afternoon rains to your advantage if you realize two things:
1. The rain cools things down
2. Lots of people leave the park when it starts raining. Of course, if you are afraid of getting soaked, you may too, but in the summer I can stand being a little wet, and I dry off quickly.
Which brings up the poncho vs. umbrella controversy. I prefer to carry a small travel umbrella. Ponchos are bulkier to carry when dry, and become a real pain to carry when wet. And wearing a wet poncho is no fun. They become heavy, hot, and sweaty. Besides, I look stupid in one.
How to enjoy WDW in August without experiencing a meltdown
Take it easy - You cannot maintain the same pace when it's 92 degrees that you can when it's 72. Walk slowly, take lots of breaks, and drink lots of water.
Get to the parks before they open - you will have an hour or so of uncrowded bliss, and be able to see more attractions in those first 90 minutes then you would see during the rest of the day. Have a very light breakfast, be there when the turnstiles start spinning, and if you want a big breakfast, look for a place in the park at around 10:00 when it starts to get crowded or go to a resort for a leisurely brunch, then go back for a little while.
Around 12:00-1:00pm, it's time to get out of the park. You could go hotel hopping, spend a few hours at the pool, or take naps, but don't bother trying to cope with the crowds and the heat in the parks. You can go back in the evening when both will start to dissipate and close the joint.
There's no bad time to go to WDW, but enjoying the place in the summer requires adjustments to your itinerary and a slightly different mindset.