It really depends on the pro.
Some are fine with it, others are not. The suggestion to check with the B&G is good advice.
I've found that generally; the better the pro, the less they will care - provided you don't get in their way.
Two quick stories;
I was at a wedding on the weekend (as a guest) and took the new 50D along for a spin. I shot primes due to the light (and to be unobtrusive) and only shot from my seat, so the keeper rate was not great. I wasn't sure if the pro had even seen me.
I crossed paths with the pro at the door of the church at the end of the ceremony. I was carrying the camera, but I smiled and said "G'Day". He half grunted, looked the other way, and kept on walking.
Now, he might be a knob on his days off too, but that half grunt spoke volumes about his professionalism (and most likely his confidence). I guess his finished pics will speak for themselves, but any pro whose nose gets bent out of shape by a GWC sitting in a pew halfway up the back of the church is not much of a pro in my book.
My sister had one of Australia's premier wedding photography firms shoot her wedding last year. Without going into details it was a $10k+ job. Obviously the pics were superb, but I was just as impressed with the photographer's demeanor and professionalism. He even encouraged and helped the army of GWCs, and made sure everyone holding a camera (DSLRs and P&S) got some nice shots.
So... coming back to my original statement; the better the pro, the less they will care - provided you don't get in their way.
Slightly off topic, this caught my attention;
x86brandon wrote in post #7436170
I have two weddings to go to in the next 6 months, both of which are very important people in my life.
There is a very interesting thread floating around here somewhere entitled "Missing the experince to get the shot" or something like that. I did a quick search but couldn't find it. Anyway; it's full of some great insights. If the people in question are truly special to you, don't spend the whole day behind the viewfinder.
Once last thing; I usually get the camera out at the tail end of the reception. The pro is usually gone, but that's when all the fun happens! It's one way to make your photos particularly unique and get some memorable shots!!