Slashdot reports that Project Gutenberg has volunteered to host IMSLP's catalog. What isn't clear is who will vet the legality of the scores, since Project Gutenberg has said it will host "as much [...] as is legally possible" and says that many of the scores mentioned in Universal Edition's cease and desist letter could also be hosted. The quote ends with "when the legalities have finally been worked out." So who will be working out the legalities? IMSLP's creator doesn't have the time or resources, that is why he closed down the database. And Tim points out all of the difficulties in investigating copyright legality. I won't get too excited until more clarification is provided.
In the meantime, Bob Kosovsky of the Music Division of the New York Public Library for the Performing Arts has provided some other sources of public domain scores:
Sheet Music Archive
Choral Public Domain Library
Chopin Early Editions
Werner Icking Music Archive
Indiana University's William and Gayle Cook Music Library, Variations
Eastman School of Music's Sibley Music Library, this service will take requests for public domain scores, putting all legal requests up on the website.