When upgrading R version, you must recompile rpy2.
Let us take a case study based on the upgrade of R2.15 to R2.15.1.
First, you need to download the source from the main page http://www.r-project.org/
Once downloaded, get the source tar ball and unzipped it.
You should have a file called R-2.15.1.tar.gz and if unzipped, a directory called R-2.15.1
cd R-2.15.1 ./configure --enable-R-shlib make sudo make install
sudo easy_install rpy2
if it does not work, get the source and compile rpy2 from the source.