The opensource project jnode.org is an attempt to build a Java based operating system with a very small native micro kernel. JNode comes with a ISO-image, which can be booted on a PC. You will be asked by the GRUB-loader for serveral options - choose the second (all plugins). After few seconds the output will be familiar - it looks like a list of fully qualifed Java classes. It seems like even the firewire drivers are implemented (probably native-wrappers) in Java. After the boot phase (about 10 seconds) just type GC. A bash-like console appears. With the command "startawt" the GUI mode can be started. It is hard to say how fast it is - I ran it with wmware, without any optimizations - the performance on my laptop was o.k.
However the JNode.org project seems to be really far. My mouse, graphic device, firewire etc (of my ThinkPad) were recognized without any problems. JNode useds openJDK - which is remarkable as well. The UI is interesting, but the server mode could become even more important. The BEA application server is capable to run on the vmware ESX, and so almost on "bare metal". jnode.org could become a really interesting opensource alternative for Glassfish or JBoss. Then appservers could ran on "bare metal" without OS overhead.