The Locomotive project provides a Mac OS X package for easy installation of Ruby on Rails, lighttpd, FastCGI, and other supporting libraries and database drivers. The roadmap indicates that while the current release is mainly intended for use with SQLite, future versions will include better support for PostgreSQL and MySQL.
The main question is how frequently updates will be released, and how easily these updates can be applied without causing problems in production environments. Manual installation of the components via DarwinPorts, while considerably more involved than the Locomotive .pkg installer, allows for easy and frequent updates to selected components.
Update: It looks as if Locomotive is a self-contained application and not an installer package. That being the case, it would seem that updates could be as simple as just replacing the application with new versions as they are released. A recent update includes PostgreSQL and MySQL bindings and adds a slew of handy new components and code generators.