We went to WDW 8/17 - 8/24 in 2003. The wether was hot and rainy with rain everyday, sometimes a half day or more. CM said it was much rainier than normal then. Crowds were tolerable with many Europeans over on "holiday". The American crowd seemed to diminish as the week wore on as many kids headed back to school.
In 2005 we went 8/20 - 8/27. This time there was no rain except for a 1/2 hour storm one day (There was this thing called hurricane Katrina churning to the south that blocked all other weather systems and gave us GREAT weather). It was stil HOT, though. Crowds started out as average, but dweindled to sparce by the last day. Our last day was in Magic Kingdom, and we did not wait more than 5 minutes to walk onto any ride.
We are going again this August. It seems the closer you get to the end of the month, the better the crowds are. Hope this helps.