When I last looked at the Mona Lisa (I try to avoid it when at the Louvre due to the crowds) there didnt seem to be a problem with non-flash photos. Have a few myself. The problem is its usually so surrounded by people you cant get near it. Maybe it was just the time I was there, it certainly wouldnt surprise me if there was some variable enforcement at the Louvre. On a recent visit I spent an hour or two in a closed gallery with a few other people. Kind of nice to look at the Donatello and Michaelangelo sculptures without the damn crowds. They had signs up saying the galleries were closed but a few people wondered around and no one seemed to care. I love France I tried a similar thing at the British Museum and was given the impression, very politely of course, that in a few more steps I would quite likely be meeting some members of Her Majesty's Special Air Service.