it seems that setting PYTHON_HOME breaks the runtime on Mac OSX. Doing so means that the system installed Python libraries cannot be found, a (large) number of which do not get installed into the virtualenv.
Now I could modify the package build scripts to copy or symlink all those extra files and directories into the virtual env (I would probably symlink, as that's what happens with the rest of the venv), but before I do - can you remind me why we needed to explicitly set it anyway?