In a few days I’m going to attend a Bioconductor Course; I was requested to install on my MacBook (Mac OS X 10.5.8) a developer version of R (plus ad hoc Bioconductor packages). In order to keep my old R installation ((2.13) along side the new one (2.14) I decided to use the RSwitch app (you can download from here) and the instructions you can read here.
In practical term, you type the following commands in Terminal:
sudo pkgutil --forget org.r-project.R.Leopard.fw.pkg sudo pkgutil --forget org.r-project.R.Leopard.GUI.pkg sudo pkgutil --forget org.r-project.R.Leopard.GUI64.pkg
You install the alternative version of R (for example, following the procedure depicted here) and then you can switch between the different version using the RSwitch GUI (see the below screenshot). So easy!