In the days of flash BULB photography, when I started working in
scientific photgraphy, flash was considered detrimental due to intense
heat given off by flashbulbs and also due to excessive ultraviolet
radiation, both of which would cause sensitive materials to fade and
otherwise deteriorate.
       Modern electronic strobe flashes, however, do not give off as much
heat and are filtered and otherwise constructed so as to minimize UV
radiation, which has adverse effects on color balance in flash photography.
It is therefore much less of a concern in terms of archival treatment of
sensitive materials.
       Casual flash photography, such as that produced by visitors to a
typical museum display, will have virtually no detectable effect on most
materials. Normal ambiant light will have far more of an effect than flash
from the average amateur camera.
        Intense electronic flash, on the other hand, such as that used for
professional copy set-ups, may indeed effect sensitive if it is repeated
often enough.  I wouldn't know where to draw the line here without
extensive experimentation, but that line could be determined for specific
materials, and perhaps has already been established.  The folks at Kodak
may have some guidelines here.
        But I would be surpised if ordinary, one-time copy stand exposure to
electronic flash would have any detectable deleterious affect on most
normal materials.
        Mike Lewis
        University of Alaska Museum