Besides the performance improvements (indeed the startup time is quite amazing compared to JBOSS, Geronimo, or Sun's appserver) they mentioned running JRuby apps and I ran across this blog entry on running interpreted rails apps within Glassfish v3 and the Grizzly connector.
I had to make few changes to get it working to Linux-ify the tutorial (had to create a symlink within glassfish/lib to point to the jruby directory) but it was relatively straightforward on Debian 4.0, (JRuby 1.0.1 and jdk1.6.0_02)
Here the slight differences:
I set the environment variables in .bashrc
Install rails (added --no-ri)
# jruby -S gem install rails -y --no-ri --no-rdoc
Modify glassfish/config/asenv.conf instead of asenv.bat
Make asadmin exectuable (chmod u+x/700)