To familiarize myself with db4o I created a small example. It doesn't reveal the full potential of db4o, but it highlights the main functions and gives an overview of how db4o works. You can run the example from the command line by passing agruments to the example class. I added a reame file for further information.
I still think db4o is a cute tool, which can be used for prototyping or building small applications. I guess there'd be performance problems, using db4o in big applications, since it's working with files.
My Example is available through blogtorrent and my download site.