I'm not sure about Gnutella. Quicksilver uses a client program on your computer to compose messages, encrypt them, pad to standard messages lengths and puts in random delays (to avoid traffic analysis), and then routes them through a whole chain of Mixmaster anonymous remailers in different countries and different legal jurisdictions. Looks like a pretty strong anonymizing system to me. I've run the client on my machines (home and laptop) and it appears to work as advertised.
More info at http://quicksilver.skuz.net
My work server firewall won't let me access it here from work. Calls it "criminal skills." Sounds like something good to research and work into a novel? ;>)