Due to the security restrictions on OS X, you can’t run anything on port 80 without being the superuser. Although running script/server on your rails application with a sudo prefix won’t go down too well as your environmental variables (gem path, rails environment) won’t be set. To get around this I usually create an alias in which I set my environmental variables for the sudo command like this:
You can add that line to the ~/.profile file in order for it to be set every time you open the terminal. This will save you having to type it in each time.