I have! A few times actually- never bad enough to where they close the parks completely down- if it was a day where it was really coming down we would just go to Disney Quest, or catch a movie at DTD. If it is just raining- we still hit the parks, just in the rain gear!
One of my best friends works for WDW, and she volunteered to work when they had "the big one" several years ago. WDW let the staff stay in the hotels on site so they didnt have to go home, and Disney just closed the parks, and did fun activities in the lobbys of the hotels- and did their best to make sure guest that were supposed to already be checked out but had flight delays still had accomodations. She says Disney very rarely closes the parks down, but they have a def plan in the event they do!