C-JDBC (Clustered JDBC) is an open source database cluster middleware
that allows any application to transparently access a cluster of
databases through JDBC.
The database is distributed and possibly replicated among several nodes
and C-JDBC load balances the queries between these nodes.


No application or database modification is needed. C-JDBC is a 100% Java
middleware licensed under LGPL. C-JDBC has been successfuly tested with
Tomcat, JOnAS, JBoss, WebLogic, Oracle, DB2, MySQL, PostgreSQL, Sybase,
SAP DB, HyperSonic SQL, *Firebird,* MS SQL Server and InstantDB.

http://theserverside.com/news/thread.jsp?thread_id=23875

ps:
I think jim is right about jdbc/java in the firebird engine
Imagine using other datasources than firebird or a jdbc cluster (of
*birds :))