With Google App engine SDK 1.5.5, Google have introduced Google Cloud SQL for GAE (Currently we can't connect any other external application with Google Cloud SQL. This can be only with Google App Engine Apps).
It's faster than connecting with Amazon RDS and other Cloud based Data solutions. No need to worry about load balancing as well. Cloud SQL supports 5 QPS read/write rate, so as long as you don't go over that, you can probably use cloud SQL. If you are going over this read/write rate limit, you will have to share your database, and the datastore might become a much more effective approach to you.
If you are storing large blobs data(images, attchments and sound files, it's better storing them in the datastore or blobstore.
When Google App Engine became populer in late 2009 many people asked about Hibernate support. But with their datasore people were unable to develop Hibernate support applications with GAE. But with Google Cloud SQL developers can use Hibernate supported applications as well.
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. http://cnapagoda.blogspot.com/2011/10/google-cloud-sql-with-struts-spring-and.html
Google App Engine and Cloud SQL will be revolutions of Cloud Computing.