So, I ask that in the same way as we have a well defined and well publicised system for using fair-use on en:WP but not on commons (or de:WP for example) we should also make a well defined, consistent and well publicised system for dealing with self-made photographs of in-copyright artwork.
It doesn't happen very often. In addition it is getting into the area of derivative works which is messy and Freedom of panorama which is lengthy. The relevant commons pages are at:
http://commons.wikimedia.org/wiki/COM:DW http://commons.wikimedia.org/wiki/COM:FOP
Self-made pictures of in-copyright artwork is derivative work and cannot be published or licensed at all without the consent of the original copyright holder. Notable exceptions include "Freedom of Panorama" (to varying degrees in different countries), and incidental inclusion, such as art printed on the t-shirt of a person that is the actual subject of the picture (subject to the court's whim).
Going into a gallery and taking a photo of a copyrighted painting does not give you the right to publish the resulting image. In fact, if you did a good job of just reproducing the painting, you may not have any rights at all to the result.
-- daniel