Java DB provides a great way to try JDBC 4.0 features
Java DB and Apache Derby have been following the progress of the JDBC 4.0 specification very closely and have implemented many of the JDBC 4.0 features in the 10.2 alpha release. Rick Hillegas has done a great job of leading the team implementing the JDBC 4.0 features.
Another key JDBC feature that has been added in the 10.2 release is support for updatable scrollable cursors.
You can find the latest status regarding the JDBC 4.0 features as well as the rest of the Java DB 10.2 features here.
Java DB will be available with Mustang later today (6/15) or tomorrow (6/16) as part of the JDK bundles for build 88, available at http://download.java.net/jdk6/binaries/