It's great to see that people share the same concerns.  A coworker just returned from the Computers in Libraries conference and pointed out a wiki devoted to primary source material called Archivopedia:  From my hasty glance, it seems like theyr'e just beginning to come up with a way of formatting EAD for wiki presentation.

As far as user-generated content, my main thought was that users would add greater detail to what the institution puts up.  For a few of our (paper) finding aids, users have provided us with detailed listings which we've been able to incorporate.  So I see providing user access gives us a chance to take advantage of their knowledge.  And of course, anything is reversible if we disagree with the changes.  It also provides a much simpler way of correcting errors.

Bob Kosovsky, Ph.D., Curator, Rare Books and Manuscripts
Music Division -- The New York Public Library for the Performing Arts
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My opinions do not necessarily represent those of my institutions.