Official JRuby wiki:

http://wiki.jruby.org/wiki/Jruby_on_Rails_on_Tomcat

Running your first Rails app on JRuby:

http://www.developer.com/lang/other/article.php/3662031

Running JRuby on Rails on Tomcat:

http://www.developer.com/db/article.php/3691716

http://adams.id.au/blog/2008/05/running-a-rails-app-under-tomcat-using-jruby/

All around presentation about JRuby on rails:

http://blog.nicksieger.com/files/Sieger-RailsDeployment-J12008.pdf

Deploying Rails to Tomcat as a WAR with JRuby:

http://edgibbs.com/2007/05/25/deploying-rails-to-tomcat-as-a-war-with-jruby

Warbler: ruby gem which creates .war out of rails project:

http://wiki.jruby.org/wiki/Warbler

Articles on warbler:

http://fuzzypanic.blogspot.com/2008/01/jruby-war-packaging.html

http://blog.nicksieger.com/articles/2007/09/04/warbler-a-little-birdie-to-introduce-your-rails-app-to-java

Benchmarks jruby on rails:

http://blog.nicksieger.com/articles/2007/10/25/jruby-on-rails-fast-enough

Multiple sites single tomcat, not about JRuby but i guess that this should work for whatever application (OLD: 30/08/2006):

http://www.onjava.com/pub/a/onjava/2006/08/30/publishing-multiple-sites-using-single-tomcat.html?page=1

Session replication in tomcat 5 (OLD: 24/11/2004):

http://www.onjava.com/pub/a/onjava/2004/11/24/replication1.html