Just a little note to let you know that I spend a day hacking a new Riak client library for Java based on the protocol buffers API.
You can find it here at github.com/krestenkrab/riak-java-pb-client.
We're currently evaluating Riak for some projects, and one of the issues we ran into is the performance of bulk import using riak-java-client. In one particular project, we have several years of data history that needed to be loaded, so initialization time was a real issue. My current tests indicate that this client library can run bulk insert at ~ 10x performance as compared to the http-based riak-java-client.
The library it still a bit rough around the edges but not entirely unusable. It's quite similar to Jon Lee's riak-java-client, and I've included several pieces of code from there. The protocol buffers API does not have all the same features as the HTTP API, so functionality is a bit different.