Raider Fan, I have had the same issue with the way the sizes on those shoes are described, too. I'd like to buy a pair of the mary jane styles to test drive for my mom (we wear the same size except she needs a wide), and I don't know what size to buy.
Almost my entire shore wardrobe is Born, and they are all sized 8.5/40. I can't imagine torturing my long monkey toes into a size 39. Or even worse, having the back of my heel perilously close to the edge of the shoe bed on a sandal.
I guess the only thing to do is find a local retailer that sells them and try them on. There are some available on eBay, but even there the sellers don't know how to describe them in American sizing. Some 39s are described as size 9, some as 8.5, it's all over the map.
When I eventually get around to testing a pair by buying from eBay, I'm going to start with a 39.