I want to start out by saying that if I am an invited guest at a wedding where I am not the official photographer, I leave my gear at home. All of it. I'm of the opinion that there is nothing more unprofessional looking that a pro/amateur shooter flying around someone elses gig taking shots.
Obviously there is a big difference between Grandma taking a few snaps with a compact point and shoot, to 'Uncle Photography' turning up with his D/SLR and an assortment of lenses. You might be trying to do the bride and groom a favour by providing them with some extra coverage free, out of the goodness of your heart. The problem is that regardless of the motivation of the individual who happens to be crashing the photography party, it just doesn't look good.
I was at a wedding recently, as a guest, and I left my gear at home, as I always do. The official shooter was doing a great job, very low key and inconspicuous, but she was getting all the shots. I was impressed with her calm and professional approach. I was not impressed by another two guys (separately) who had turned up with their D/SLR's and L lenses (both currently trying to crack into the pro market). They were up on chairs, up and down the aisle, one was even trying to get the wedding party's attention for a shot at one stage.
I've seen this type of scenario happen time and time again, and it simply looks terrible.
Can I please plead with you all, if you are a wedding photographer (or even aspiring to be) and you're an invited guest at a wedding, please leave your gear at home. You're only damaging your own reputation, because it makes you look unprofessional and desperate.
You might even enjoy the ceremony more, too. 

_______________

