I liked the wedding photo with the bridesmaids in chocolate brown (and Paris Hilton with a blue face), but I think brown for formalwear is very tricky. One thing I believe about brown formalwear (for women, anyway) is that brown is a very informal colour, so the cut and fabric can be überformal then. A classic formal gown made of chocolate satin can be very nice (plus you avoid being one of the hundred women at an event wearing black). I’m having a hard time imagining a brown tux looking formal enough, though. Brown suits just aren’t formal enough, no matter how many satin lapels you put on them.
As an aside, if I had brown skin, I don’t think I would wear any colour except brown. Man, that looks good!