Usually I wear dress pants and a dress shirt; no jacket or tie. Though I always bring a simple dark tie just in case there is a uniform police officer at the wedding, I have never lasted with one for more than half an hour of "serious" shooting, due to camera straps getting tangled and what have you. (If I must wear a tie, a bow tie it has to be. ) For more informal weddings I wear ironed work pants (e.g. Dickies) and a short-sleeve button-down shirt (usually one of my vintage '50s numbers that I get in thrift shops as they come up).

As always, the rule of thumb is that it is better to be over dressed than under dressed.