On the Media had a great piece on The Uncanny Valley, a theory that explains why we get creeped out when a robot, a doll, or a 3D computer character, or (oh dear), a portrait, looks too real. You know that feeling of wrongess when characters look almost real, almost human, but there’s something not quite right. That’s the uncanny valley.
One of the fellows interviewed draws a parallel to 19th century realism and posits that this response to ultra realsism is why the art world left it behind. I don’t know that Bouguereau’s paintings have ever creeped me out, although sometimes I find them a little goofy and sweet.