There is also a directory src/samples-resources where you can put various files (property files, testing data etc.) that should be found by the sample code. All these files will be packed together with the samples code when Ant's target samples-jar is called.
Additionally the org.biomoby.shared package is a good place for putting there Java interfaces - assuming that they are expected to be used more generally. For example, here is Central.java - an interface defining an access to a Moby registry.