Yes, an Orbiter written in java would be nice, but it will take a lot to implement and maintain. The big problem will be to keep C++ sources and java sources in sync (adding a feature in C++ will need same modification in java code). Also, adding/modifying a command/event to Orbiter will need a modification made manually in java code to adapt DCE; so a code generator for java would be needed too.
For an Orbiter Mobile, we'll need to port BD commands from C++ to java and keep them in sync, but this will be easier that an "desktop" Orbiter.
Anyway, I would gladly support an Orbiter written in java, so if you need help for this, I'm here to help.