The American National Archives and Library of Congress both allow the scanning of documents with some scanners (see what's allowed and what's not). FlipPal is one of their approved ones. Lorine McGinnis Schulze has also had great success doing personal scanning at her local archives (Olive Tree Genealogy blog). I've recently purchased a FlipPal and love it for home use, but now I want to start bringing it along on my research trips to repositories and am intimidated by the many organizations that have a blanket rule "No Scanning."
Is there something that the National Archives knows that others don't? Wouldn't they be one of the best judges of whether something is harmful to the records or not? Or, are they just weighing out the benefits vs. risks with letting people use these scanners... Like, increased access to records in exchange for the risk of some damage accruing to the record?