I know this may sound slightly silly, but I've heard that it can work reasonably well elsewhere...

How about each client, when asked to upload a file, also get's a chance if it decides (for whatever reason, say for example, it's never uploaded to this IP address before) to upload a message that has to be OK'd before the upload commences.

This message could be something along the lines of 'I agree that I do not work for any law enforcement agency, nor for trade organizations such as the WIPO, RIAA (etc etc) and that I agree to indemnify both the owner or owners of the remote computer currently identified as having IP address x.x.x.x from any legal action as a direct or indirection consequence of this or any subsequent download download that I, or the organization I represent, perform. I further agree that I am solely responsible for ensuring that any and all copyright infringements are not violated by performing this or subsequent downloads and that I/we do not hold the owner of the remote computer responsible in any way, shape manner or form.' (Yada, yada, yada)

Long winded? Yes. Turgid? Certainly. Protective? Possibly...

Perhaps something like this would then put the onus on the uploader to ensure that no copyright infringement takes place. Since the uploader is significantly harder to ID than the sharer, this might make the job of the RIAA harder to perform...

Just an idea is all. Perhaps my head is somewhat in cloud cuckoo land here, but you never know...

Rachel