There's no lock-in on the Java side. Change a few lines in jdconfig.xml, add the datanucleus-rdbms jar file, add a JDBC driver jar file and you're app will run on Tomcat or Jetty with MySQL or any database with a JDBC driver.

Check out the AppScale project: http://code.google.com/p/appscale/

You can run your App Engine apps pretty much anywhere.