The last few versions such as 9.1 come bundled with a ReadMe that is misleading. The ReadMe states that you need to adjust your shared memory configuration. Actually, the installer does this for you.
 Download and run the installer.
 Read the following message displayed by the installer.
The installer installs some script to automatically launch the Postgres engine when your Mac starts.
 Run the installer again to complete the installation process.
I am recording here the last last sentence in that message, for future reference:
If you wish to restore the default [memory configuration] settings in the future, simply delete the /etc/sysctl.conf and reboot.