Worried that you'll be the only guy (or woman on the arm of said guy) at the event to show up in a run-of-the-mill suit and tie?
Don't sweat it! Obviously, the bride wouldn't have tacked "optional" on the end of the phrase if she didn't mean it. If you're worried about looking shabby next to other guests, do a bit of friendly reconnaissance to determine your wardrobe options. A quick phone call to your mother, aunt or even the couple in question is usually all you need to ease your mind about such questions. Especially if you're friends with the groom, he'll come clean about the bride's expectations for attire.
If no one is offering specifics about wardrobe or they're laughing off your concerns, it's safe to say that you can show up in something other than a tux. According to wedding etiquette experts, men can sport a conservative, dark suit and tie and still meet expectations. Women can opt for a nice dress (read: nothing too short) or even a skirt and top combo.
A note to couples: Be certain to get your ducks in a row ahead of time. A woman who dresses up in an evening gown for the event will be pretty put out if her date wears much more casual attire.