Olivier Fréchard has an interesting and practically useful article at Focus Numérique on how to manage those who bring their cameras along to a wedding and frequently get into your way as you try to immortalize the event.
It’s in French but you can easily enough get Google to translate it for you.
Advice ranges from the need to recognize that, even when you officially make the request for guests not to get in your way, some still will, so accept the situation and be prepared for it. Get the bride and groom on your side and they can pass the word around to their families and friends. Interesting observation: it is those with the DSLR that you should be most concerned about; they will have no compunction getting up and moving around to catch the best shot — and stand right in front of you. Let the guests take the group shots first. Always be polite on this beautiful day for the married couple, their families and friends.