The neckline has been revisited by more than one designer recently, with tweaks to the depth of the sweetheart, width on the shoulders, etc. Both Queen Mathilde and CP Mary have worn variations. I think what makes the version worn by Grace amazing is the way it sits on the shoulders, which I don't think I've seen replicated, not to mention that gorgeous cinched in waist.
The Alexander McQueen version worn by Catherine is very sleek and lovely, I especially like the simplicity of the sleeves and the drape of the skirt.