Just tested it with trunk. Most of the time I see the same behavior (hangs), but on 2 runs I got the RingNotFound error. That happened after I'd run the same program (list_services.rb) with ruby first. But I haven't found a scenario to reproduce it. There may be some environmental things in play here, but I don't really know what to look for.
Another thing that's changed with trunk is that I can now start the ringserver.rb example. This used to fail* in 1.0.1. Ruby list_services.rb does not detect the ring though.
*) /home/menno/develop/jruby-1.0.1/lib/ruby/1.8/rinda/ring.rb:34:in `initialize': uninitialized constant Rinda::RingServer::UDPSocket (NameEr/home/menno/develop/jruby-1.0.1/lib/ruby/1.8/rinda/ring.rb:34:in `initialize': uninitialized constant Rinda::RingServer::UDPSocket (NameError)
from :1:in `new'