Over the years I have only covered 5 weddings, I find them an absolute nightmare.
At any important event always go and ask the organiser. If he/she has any special requirements they will tell you! Have an in-depth discussion with the bride and groom, it is usually their local church and then discuss their wishes with the presiding priest.
Enlist the help of the best man if there is one, it can save a lot of time sorting out the in-laws and relatives.
Paul Owen stated that pictures during the register signing are not permitted, this varies from priest to priest in my experience. Some if they are fairly liberal in their approach, allow a number of pictures, if flash is not used, so make sure you have some fast film. You can always argue that if they permit video you could surely take stills without flash.
Do not under any circumstances take a picture that is not permitted by the priest, not only does it result in bad feelings between you and the priest, you might be offered another wedding in the same church, or he/she could ban other photographers completely!
Work hard beforehand in ensuring that everything is double checked!