If you have production servers running jruby/rails applications packaged as war files and dont actually have the full jruby installed to run your migrations, you can follow these steps. This will allow you to run db:migrate without a jruby installation on your java application servers
Assuming you are deploying your app called 'myapp' into tomcat5, you would have a warfile located in $CATALINA_HOME/webapps/myapp.war. if your war is not unwar'd you can run "jar xvf myapp.war" to generate the unwar'd directory of your app (which you will need)
# cd $CATALINA_HOME/webapps/YOURAPP/WEB-INF
# export GEM_HOME=$CATALINA_HOME/webapps/YOURAPP/WEB-INF/gems
# cp $YOUR_REPISTORY/Rakefile .
# mkdir -p db/migrate
# cp $YOUR_REPISITORY/db/migrate/* db/migrate
# java -jar lib/jars/jruby-complete-1.3.1.jar -S rake db:migrate