Taliban: In what way? Digital signatures protect the integrity of individual ratings, and if each user maintains a list of public keys that they trust, they can filter out ratings they don't care about. The main problem with storing ratings on Gnutella is that individual ones might be hard to find.

NiGHTSFTP: Searching a local database can be made pretty fast, as long as you filter out the junk. But you're right about propagating the little rating files all over the network -- that might be bad for bandwidth. A compromise solution might be to use a protocol that "syncs" two users databases. There are efficient protocols for this that use hash trees.