Over the past few years museum curators and optical researchers have begun using spectral analysis and other imaging techniques to study the great masters.
I wrote an article about it recently as an online-only feature for SPIE Professional Magazine. SPIE has put on several conferences under the Electronic Imaging Symposium umbrella that have dealt with this same issue. A 2006 conference put on by Boston PBS station WGBH and Museum of Science, Boston recorded their proceedings and is available to view here.
The benefits of this type of study are that you can virtually tear apart the painting without damaging it.