super easy

gem install passenger
passenger-install-apache2-module

will walk you through the install and remember to copy paths to your apache config file. for passenger i kept it in mods-available/passenger.conf and then linked it to mods-enabled

ln -s /etc/apache2/mods-available/passenger.conf /etc/apache2/mods-enabled/passenger.conf

then just restart apache

more info and official docs at http://blog.phusion.nl/2009/03/13/phusion-passenger-212-final-released/

Just finishing up brewing up some fresh ground comments...