Here is a cool post about getting Rails (grails) running on AppEngine under Java.
http://blog.inigral.com/2009/05/07/rails-on-app-engine-a-cold-start/
This shows the vast potential of the Java AppEngine Java environment – because the JVM is really just a virtual computer – it can do anything in any language. The only consequence might be performance – but over time any linear performance factor fades away as a real issue.
Thanks for the link Michael.