This is only the second release after 0.1-0 more than six months ago. Given that Rcpp is such a key ingedrient for RProtoBuf, and that Rcpp underwent so many exciting changes itself, Romain and I never got around to releasing new versions of RProtoBuf. This version is now much closer to the actual C++ API and fairly feature rich. We summarised a few of these new things in the presentation at useR! 2010.
There is more information at the RProtoBuf page; there is a draft package vignette, a 'quick' overview vignette and a unit test summary vignette. Questions, comments etc should go to the rprotobuf mailing list off the RProtoBuf page at R-Forge.