I would encourage everyone working on drivers for RethinkDB to publish them regardless of how complete they are, and advertise them as partial implementations. I know of a lot of people interested in working on a Java driver, and it would be great to see collaboration, even early on!
As far as a JVM-based client driver, kclay in this thread mentioned that he's been building a Scala driver that is compatible with 1.5, and will update it for 1.6. There's also a thread on Google Groups (https://groups.google.com/forum/?fromgroups#!topic/rethinkdb...) detailing some efforts on a Java driver to date.
If you're interested in contributing, shoot me an email at mike [at] rethinkdb.com. I'd love to see a Java driver happen, and we'll do everything we can to support such a project.