Originally posted by jimhi but I have a son getting married this fall
My wife and I have been the (non-offical!) backup/alternate photographers at three niece/nephew weddings. In two cases, I really was a backup (i.e. they knew what they were doing), but in the third, I think I was as good/prepared as the "official" photog was! And, your shots will be much different from the official ones, I made a point of shooting background and crowd/group pictures that might well have been overlooked by the official guys. I think it is many of these shots that will be appreciated/remembered long after the event.
If anything, I would suggest you get into digital, where you can shoot to your heart's content, withour worrying about film costs/printing.