It's written in Java, so I'd guess it won't be easily embeddable in things not written in Java. So I'd guess there's not that much overlap: if you're writing in Java and want an embeddable database, then cloudscape is going to be useful new option; otherwise sqlite will be one of the databases to consider.
Just want to point out that there is already HSQLDB for java-embedding, it is a very successful project, easy to use and very reliable. Available at sourceforge.
SQLite is of course not really affected by these tools as it runs as native code. HSQLDB is however surprisingly effective and easy to use if you are looking for an SQLite equivalent for Java programs (around 100Kb for embedded classes)
Regards, Mauricio Tabuleiro
