I wouldn't be an official wedding photographer for any amount of money ... too much pressure (for me), and too many amateurs with P&S cameras or cellphones flashing away over your shoulder.
That said ... I'm an enthusiastic unofficial photographer at many weddings I attend. I'll only take one body and one lens, either a standard zoom or a fast prime in the 85-100mm range.
I keep my camera at the ready, but only shoot during the ceremony (from my seat) if others are, as well. If no one besides the official photographer is shooting during the ceremony, then I don't either. However, if there are P&S and cellphone cameras clicking and flashing throughout the audience, I'll take my shots without guilt (and without flash).
During the posed shots after the ceremony and during the reception, I'll shadow the official photographer discretely, and only take my shots (with flash, as appropriate) if / when he gives permission after he has gotten his shots. If he doesn't allow it (or ignores everyone else), I let him go about his business unmolested.
Never had an issue, and have come away with some great shots. I always send a disc of processed images to the newlyweds, some of which have been preferred over the official photog's offerings.