In my last two trips to WDW (October 2007, August 2008), it rained off and off during both of my visits.
October wasn't too bad. I went to Mickey's Holloween party on a Sunday nght and off and on it rained, and rained well into the evening. But it was quite humid for mid-October (about 10 days before Oct 31). It remained pretty warm and humid through Tuesday (88-92) before a cool front and some rain dropped the forecast into the upper 70s and low 80s with mainly overcast skies. I was told by a friend of mine during my Oct 2007 that the mid-October conditions were unusual.
As for the August 2008 trip, as luck would have it, most of my trip I spent dating Tropical Storm Fay. Was there Monday - Saturday third week of August. First day - 90, high humidity, sunny and awesome. A brief mid-afternoon storm settled things down and the evening was clear and in the 80s. Next morning through Friday, Fay arrived and made her presence known. Took it in stride visiting Universal, SeaWorld and Disney. Still wore shorts, but lugged around the rain poncho. Sudden downpours, sideways rain, heavy winds. Pretty sweet. By late Friday she finally pushed off toward Daytona and Pensacola near the panhandle and by Saturday it was clear, upper 80s and humid. There was another gullywasher mid-day but once it cleared up, very nice conditions.
But if it wasn't for TS Fay, she would not have given me this or this
Although I don't live in the south and sometimes only see renmants of hurricanes and tropical storms if they push through the Midwest, so I'm no expert on Florida weather.
Crowd levels were minimal both times I visited Central Florida. While I played in the rain with Fay, I heard other stayed indoors. As for October, crowds were very light at all properties, with the exception of heavier attendance late Thursday and Friday, when the weekend peaked.
Best of luck!