I can see powerfull light (any part of the spectrum) as well as smoke and polutants damaging artwork, but I question how much damage photographers will do, with their flash and IR light.
Really, from what I understand, museums do not want you to take pictures so that they can sell you postcards. Also if you show your pictures to people then they may not need to actually visit the museum, though I find that showing them pictures makes them want to visit the museum more.