Here are some ideas I posted over at bearshare.net:

I think a blend of gnutella and (the idea behind) freenet would be best. Users would be able to share files on their harddrive, unlike freenet, and would also be required to have at least 100mb for encrypted files that would be shared. To any host searching/downloading they would be unable to tell if the file was one that was encypted and the user was unaware was even on the drive or one that the user was sharing. All network traffic should be encrypted. When encrypted/mirrored files are stored there would also be a separate file the user would have with the file name, meta data, and key. This would be searchable. So clients would also be creating lists of available files, meta data and keys for searches.

Some fraction of uploads should be rerouted. All of this would make it impossible for anyone to every say that a particular person was sharing a particular file.

Please see the following discussions for more:

http://bearshare.net/forum/showthread.php?threadid=4266

http://bearshare.net/forum/showthread.php?threadid=4938