Effectively there is a port of a Java Virtual Machine which runs on top of Mono, and uses the GNU Classpath libraries to provide the standard Java classes (such as java.io.File etc.)

It's not that big a think as a direct port of a JVM to Maemo shouldn't be too hard - and using the GNU Classpath libraries will get a good degree of compatibility.

That'll allow some degree Java support for Java-based applications, but is still a long way from Java applets embedded in web pages.