I've gone to WDW in August four times and I'm going again this August. It is very, very hot and very, very wet. I actually bring two pairs of sneakers because inevitably I'll get caught in the rain with one pair. I bring flip flops as well to wear around the resort.
Funny story about the first time I went in August. My wife and I live in New England and thought, "well, it will be hot during the day but we should probably bring a sweater or two for when the temperature drops at night." Ha! What a bunch of idiots! Needless to say, the temp drops... from like 98 to 88.