We always tried to avoid summer on our WDW trips -- we were very fearful of the heat and crowds. We had done 4th of July, and compared to Oct., or Jan, or even the week before xmas, it was just too much humanity for us (and all sweaty ones, too).
HOWEVER... along came our 3 kids, and now, with school and sports schedules such as they are, literally the ONLY time for our last trip was August. We took the 3rd week timeframe, for 9 days. By that time, many of the Southern schools have resumed (ours started the day after we got home!) Personally, yes it was hot. We thought AK was the worst for feeling that heat (tho' Kali helped some!) And it was humid, and it did rain for a brief spell each day. But on the flip side of that -- the kids used the pools ALOT and LOVED their first trip to a water park (never did a water park before -- our trips in the "off-season" always seemed too cool... our last Jan trip it never got above 50. I know the pool is heated.... but sooner or later you have to get OUT of it! Brrrr!) The crowds were not bad at all, either. And with summer clothes, you sure don't have to pack as much (no jackets or long pants needed!) We actually have scheduled a trip for Aug 2004, so we must have enjoyed it! The kids are very excited about hitting BOTH water parks this time, and maybe finally getting to try Disney Quest -- all good ideas when you're beating the heat!
My only side-bar to this, however, is that the kids get so dehydrated so quickly in the heat. Plan to take lots of breaks; if they are still nap-aged, head back to the resort during the heat of the day. I can't imagine carrying a baby in that heat, either -- a hot little potato pressed up against an already hot "you" is bound to be less than comfy for both parties.
See you in August 2004, maybe??