Mark,
You need to discuss their expectations before the wedding. My suggestion woud be to clarify this as soon as possible.
I wouldn't be as concerned about possible problems as some here want to suggest is inevitable. The only way that can happen is if their expectations and your delivery do not meet.
My suggestion would be to start by offering them your time and talent for their entire wedding day. I would also include a DVD of all their full resolution digital image files. If they want something more, like a set of proofs or an album, this should be discussed prior to the wedding.
Rick
