This cleavage discussion is quite amusing ...
First of all, I think it is obvious that Letizia's clevage is "man made" rather than natural. It is obviously not surgery, unless there is a form of breast enhancement now that can be done in a mere few hours, including recovery. When she wore the white and beige suit earlier in the day she was not so busty ... yet a few hours later she suddenly is double her size? Obviously it is a push up bra or a padded bra or both.
Secondly, how much of her cleavage is really visible? From the pictures I see here the blue sash covers more than half of it. It is more the illusion of her cleavage that is visible beneath the sash and the bit of skin that is showing that the sash does not cover.
Thirdly, should she be wearing a turtleneck? She is a young woman with a good figure, so why shouldn't she show it off a bit? Her chest is not hanging out nor is it falling out of dress. I think what little of herself she is showing is relatively discreet when you consider that there are pictures of Stephanie of Monaco baring much more than this at the Rose Ball, or when you consider the red dress Madeline wore to the Nobel Prize one year.
It's a shame that the focus is on her cleavage so much here when it really is not that much of an issue. Most people probably see more cleavage and skin on television in the middle of the day than this.