It looks like Rachel Bilson has got a nice piece of jewellery in the form of a nipple ring in her right boob (the left one as you are looking at the picture). Now I've looked at this picture and am quite positive that its not detail on the dress or anything. And if anyone can spot a nipple ring, it would be me as I sport one too! I love that she's not afraid to adorn her body with a piercing and that she isn't someone you would automatically think of when you think about pierced nipples!
Click the pic to see a larger picture to make your verdict about it - I've even ringed it so you don't get lost!