It's an idea that bounces around,
here is a 2006 version from someone who buys into a lot of hidden meanings in the paining and considers important the fact that Mona Lisa is an anagram of Mon Salai (which kind of ignores that the painting didn't get the Mona Lisa name until 30 years after da Vinci died).
Also, the Louvre says that Vincenti (the expert that claims the hidden letters in her eyes in this story) has never actually had anything more than a tourist's access to the painting.
So this just seems like more Dan Brown-esque flimflammery to me.