$ rvm --create use jruby-1.7.4@rails3
$ alias rails3_env='export IS_RAILS3=true;rvm jruby-1.7.4; rvm gemset use rails3'
$ alias rails2_env='export IS_RAILS3=false; rvm jruby-1.7.4; rvm gemset use global'
$ alias rails3='rails3_env; script/server3'
$ alias rails2='rails2_env; jruby script/server'
$ rails2
$ rails3