On the Mac, I needed to patch rpy2 2.0.8:

rinterface/__init__.py:
- R_HOME = os.popen("R RHOME").readlines()
+ p = os.popen("R RHOME")
+ time.sleep(1)
+ R_HOME = p.readlines()

robjects/__init__.py:
- import rpy2.robjects.conversion
+ import conversion