In Last weekend I got a time to focus on this idea. So finally I have deployed my Struts, Spring and Hibernate based application in to GAE and it's data source( Database) is Google Cloud SQL. If you are migrating your existing Spring Hibernate based application in to GAE and Cloud SQL, there is not much to do.
Full Source Code(with all Jars): http://code.google.com/p/cloudsql/
Hosted Example Application : http://mycloudsql.appspot.com/
Blog post: http://cnapagoda.blogspot.com/2011/10/google-cloud-sql-with-struts-spring-and.html