Stumbled across this early this morning: Why the Mona Lisa's smile is so strangely alluring, and seems to come and go. From the website of Harvard neuroscientist Margaret LIvingstone:
The elusive quality of the Mona Lisa's smile can be explained by the fact that her smile is almost entirely in low spatial frequencies, and so is seen best by your peripheral vision (Science, 290, 1299). These three images show her face filtered to show selectively lowest (left) low (middle) and high (right) spatial frequencies.
So when you look at her eyes or the background, you see a smile like the one on the left, or in the middle, and you think she is smiling. But when you look directly at her mouth, it looks more like the panel on the right, and her smile seems to vanish. The fact that the degree of her smile varies so much with gaze angle makes her expression dynamic, and the fact that her smile vanishes when you look directly at it, makes it seem elusive.
Livingstone has done some other fascinating work on depth perception in artists; check it out at her web page.