I am not as good with words as Rachel, but here are my two cents:

Have the legal notice as part of the Gnutella License agreement (have servants do this during install?). The agreement would absolve all servant's users from responsibility saying something like:
"By using the Gnutella network, you agree to the following - You may only download file(s) that you are legally entitled to own. The Gnutella network is here to share files for educational purposes, and for the legal transfer of information. If you are downloading a file, you are stating that you legally own a licence for the product, you know the software is freeware.... bla bla bla".

My servant could be set up to varify that you have "Pre-Agreed" to the generic disclaimer before I send a file to you. I think this "blanket" disclaimer would be easier than having a disclaimer pop up every time I want to download a file from another server.... just say "I agree" once and be done with it.

Furthermore, this will also keep abuse down. Because if every user sent a disclamer, it won't be long before some smart a** starts sending 'check out my porn site' notices when you try to download a file.

In general, I think the discaimer idea is a super idea though... just need to figure out the bugs???