Thank you for your helpful answers - I will get her the 36D and if it doesn't fit then the store will give her credit to buy something else.
Here's a picture of the bra in question
http://www.annsummers.com/single.asp...cat=1&pid=2789 - she saw it in a magazine and thought it was cute (I took that as a hint), and she was disappointed they don't do DD cups. The net bit apparently unbuttons to reveal nipples a la surprise, so I guess getting the slightly larger 36D is the lesser of two evils.
Thanks again.