There’s a post at Imaging Resource, Does flash photography really damage art? The persistence of a myth, in which the author cites a work online, Amateur photographers in art galleries: assessing the harm done by flash photography. Pretty much both (the second scientifically, the former by talking to security guards) claim that flash photography does no long term harm.
I have to wonder if either actually looked at the data.
If you hit the link to the original article by Evans, you find that the testers reduced the flash output from the Metz 45CT-1′s to about GN 20 at a distance between 1.1 and 1.4 meters, to approximate exactly what an amateur flash would put out. (remember, those little flashes also have a fairly short range, and many P&S cameras have wide angle lenses that inspire the “photographer” to be close) After the test, about 1/2 of the pigments showed changes “visible to the eye.”