I'm definitely not an expert, but I have been looking at the scalability of RoR as well. Twitter was built with RoR and you can kind of take that as a good thing and a bad thing. They had some trouble in the beginning, but from what I can tell they really didn't consider the scalability when they built it. I think if you plan to have to scale then you should be fine.

A couple of things to look at: http://www.buildingwebapps.com/articles/6419-can-rails-scale...

http://rubyonrails.org/applications