Don't Block the Shot!
I recently read a story from a wedding photographer about an important shot she was taking that was ruined by someone with an I-Phone trying to get their own shot. She brought up a good point that the I-Phone user probably wasn't going to use the photo, whereas the bride certainly would have, so several parties were actually injured by the thoughtless act. When you're there as a guest I know it's tempting to want to capture every moment of the experience you can, but don't do it at the expense of the photographer who is getting paid, and the bride and groom who are doing the paying.
Live in the moment. Enjoy the full vista of the occasion instead of viewing it all on a tiny screen. If you simply must have photos, be discreet, stay out of the way, or, better yet, arrange with the bride to get copies of her professional ones.