So I know we briefly discussed this before but I wanted to take a look at the possibility of using Rails over PHP for a moment. I took some time to put together some resources that might make you change you mind on this issues. These resources cover some basics on the PHP vs Ruby discussion that have been happening elsewhere, Ruby and Rails for the PHP developer, information on Rails scalability, and a rather larger list of hosting companies that support Ruby on Rails applications.
One very interesting comment I saw that comes from one of the case studies below is that an online community recently moved their application from PHP to Rails and the code base shrunk from 50,000 lines of code to a mere 5,000 lines of code. Not only that but the site severs over 1 million pages a day, not too bad.
Please take some time to look over these as I believe it would be in the best interest of this project to at least look into the possibility and understand the benefits and drawbacks of a Rails approach.
Study finds PHP developers likely to start next project using Rails
Ruby for PHP developers
Rails for PHP developers
The lighter side of Rails vs the world...
Good overview of PHP vs Rails
3 Part discussion on moving to Rails from PHP
A Good Rails slide show
A nice starter list of Rails hosting sites (Google finds more...)