This hasn't been promoted heavily (that I recall) nor do I know who the
individuals are who are involved with coordinating this, but it looks
like they're looking for additional institutions to participate. It's a
very nice site (IMHO) - so check it out.
The OldMapsOnline Portal is an easy-to-use gateway to historical maps in
libraries around the world.
It allows the user to search for online digital historical maps across
numerous different collections via a geographical search. Search by
typing a place-name or by clicking in the map window, and narrow by
date. The search results provide a direct link to the map image on the
website of the host institution.
http://www.oldmapsonline.org
Angie
AGS Library
Milwaukee