Be prepared for anything.
In 1999, we were there from 10/31 thru 11/6 and the weather could not have been more perfect! It drizzled ONCE for an hour or two and was chilly a couple of mornings, but all afternoons were gorgeous and perfect for swimming or water parks.
In 2000, we were there a bit later in the month (I think around the 15th -- isn't it terrible that I forgot already?) and the weather wasn't as great. It drizzled on us a few times and was quite a bit chillier. In the 5 days we were there, it was never good for swimming. A couple of the days were quite nice, but it was darn cold all the nights and early mornings. Still, it wasn't nearly as bad as our trip this February, where it rained on us EVERY SINGLE DAY and never got above 50 degrees.
Bring lots of layers. (Or use it as an excuse to buy Disney sweatshirts.) Have a great time!