Ted Gocek has developed a Blender to Renderman pipeline. His method is not for the faint of heart as he's using a host of (open source) tools to achieve his goal. Still, if you need Renderman then this may be useful for you.
This method works on Windows, but Linux and OSX users may encounter problems with CgKit and PyProtocols.
I have posted a wiki article on how to create a decent WORKING Blender to Renderman pipeline on the Aqsis Wiki.
While this is mainly a start, it took about a year and a half to overcome many hurdles, and most of the credit goes to Bobby Parker's Neqsus/BtoR plugin. I just created a process with additional tools and methods. This pipeline is geared towards individuals and small studios with limited funds and experience, but want to use free or open source 3D and Renderman software.