In the last few years we've been to Disney the first and last weeks of October. Both times we did have some rain. It didn't rain everyday, but it seemed like when it did rain it lasted at least a few hours, unlike the summer 30-90 minute storms.
Overall on all my fall trips the weather is by large really nice. I would be prepared for rain though because it's better to be safe than sorry. It will also be much cheaper if you come prepared as opposed to having to buy it all in the gift shop.