I've been reading about all the RIAA subpoenas and it occurred to me that you could introduce a "reasonable doubt" of copyright infringement if your P2P also did *periodic* anonymous proxying. The important point here is that it doesn't necessarily have to be a full-blown proxying system (like FreeNet) with all the performance penalties therein.

Basically, I'm imagining something like this:
- connect to Ultrapeer(s)
- obtain a short list of other nodes
- ping other nodes to see which ones are "close" (in network terms) and to inquire about proxying capability
- establish a co-proxy relationship with 1 or more other nodes
- direct every 10th(?) PUSH request to one of your proxies, along with the requested file

Now, if some enforcement agency comes after you, you can argue -- "I wasn't distributing that file, someone else was and I must have proxied it for them." Since the basic principle of P2P file-sharing seems to be legal, I think this raises some legal doubt about whether or not you are culpable for the detected infringement. Whether or not it's a "reasonable" doubt, however, is another matter.