I have nothing against RoR, but I don't think it is a good choice for the goal of having an easy self-install application. If you look at the popular self-install apps, such as Wordpress, PHPBB, Drupal, etc. they are all written in PHP. There is a lot more small-scale hosting support for PHP, especially because of mod_php. Dump all the files in a public directory, fire up the URL and presto. While there are some RoR hosting providers who can do this, most of the target audience (ie. semi-literate tech people) are more accustomed to being able to install a PHP app.

I use RoR, but if I had to build a product that users would install on a server and run themselves, I would probably build it in PHP, despite the other known disadvantages.