It's never a good idea to wear anything that could conceivably be mistaken for a wedding dress. But, just like the tradition against wearing black to on the big day, we think this rule can be broken -- if you go about it the right way.
A simple, off-white dress or skirt should be fine, and if you want to be absolutely sure you're not stepping on any perfectly manicured toes, get a glimpse of the bride's dress before the big day. Make sure your white outfit of choice is dramatically different, so if she's wearing a full-skirted gown, your slender, white evening gown shouldn't pose much of a problem. If she's wearing a simple number, though, pick another color. Even if you look your best in white, you won't care what color you're wearing when she's staring daggers into your eyes instead of saying "I do."