I was just going to post this question Jim. I'm curious to know your answer!
I tell potential clients that I am a member of a professional organization, the Wedding Photographic Society, who's mission is, among other things, to provide capable, professional photographers as backups whenever a member is sick or injured. While I've never missed a wedding yet, there have been others who needed a backup at a moments notice, due to car accidents. The WPS was able to provide a photographer to cover the wedding within an hour.
The WPS keeps a list of emergency photographers just for this reason.