Disney will certainly assign you a back-up location for any outdoor parts of your wedding events. But like hmgolden said, you'll want to make sure you will be happy enough with your back-up choices. If you are really nervous about weather, another idea is to have one of the "lesser" events of your wedding outside, while keeping the ceremony and reception indoors. For example, we had photos and an Illuminations dessert party outside.
Good luck picking your locations!