According to
http://www.livescience.com/widgets/birthday.html - "You may have thought your sign is Sagittarius, but your true sign is Ophiuchus." In other words, that is the constellation the sun was in when I was born. The friend I found this through actually came out as her traditional horoscope sign. So far, hers is the only birthday to come out like this of those I've tested so far.
My daughter's results - "You may have thought your sign is Capricorn, but your true sign is Sagittarius." She's probably going to be happy about this, until she finds out that it's true for her dad too.
My son's results - "You may have thought your sign is Scorpio, but your true sign is Virgo." He's going to be annoyed. He likes claiming that he's a Scorpio.
Anyway, back to Ophiuchus.
Here contains probably more than you want to know about this controversal sign as far as astrology goes. The name itself means "snake-holder".