« Lab rat: Swapping gets legit » in the Red Herring discusses a system called LOCKSS.
LOCKSS mirrors the way the librarians share their print archives. Each library keeps its own collection but borrows from the others to make a copy if a particular publication is missing or replaces it if damaged. If a library wants to join LOCKSS, it only needs the software client installed on a PC and a network connection to both the library’s local network and the Internet. Once installed, LOCKSS automatically creates a permanent cache, or local database of the journal articles. Unlike other caches that are deleted after a day or so, the LOCKSS cache is never deleted. However, it can only store copies of the journals for which a particular library has valid subscriptions.
Interesting application of a P2P model. It does raise a number of questions, regarding IP but also regarding integrity of the content and auditing. I guess the advantage of a P2P model here is that it avoids the need for the central authoritative server used in a mirroring scheme.