Look for Java DB
The first person to find the treasure wins it.
Lance Andersen, JDBC 4 spec lead, has pointed out the JDBC 4 capabilities you'll find there.
David Van Couvering has written a blog entry which explains where to find Java DB in the Java SE (Mustang) distribution and the limitations regarding upgradability of databases created with alpha versions of Java DB.
Francois Orsini's blog entry on the bundling of Java DB with Mustang provides a broad review of Java DB technology.