The JXTA site's tutorials section includes a really nice 140-page PDF tutorial for people just getting started on the subject: Project JXTA 2.0: Java Programmers Guide.
Before beginning the column, I had no idea that JXTA was language agnostic. The only thing it has to do with Java is the fact that the reference implementation -- a 528-class, 88-package monster -- is written in the language. JXTA's a group of related XML dialects that could be implemented in C, Perl, UserTalk, or favorite language here. Peers don't have to be running the same implementation -- my Java-based peer could form a group with your peer written in Kvikkalkul.
The subject gives me an idea which is probably being implemented somewhere: At some point, the popularity of site syndication formats such as RSS is going to hit a scaling wall, because it's inefficient for thousands of people to request a feed 14-16 times a day just to check for changes.
I think a cloud of JXTA peers could be the solution: In a group of peers, ask if anyone has a copy of a particular feed stored within the last hour. If they do, request it from them instead of the publisher. Otherwise, go get one yourself and cache it for others. A client-side aggregator such as Radio UserLand or NetNewsWire could form a peer-to-peer network of its users or a larger network of any RSS-consuming software users could be created.