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.
Fantastic, thanks! This is right on time for some of the discussions we are having and I really appreciate your taking the time to point these out! Would love to hear if you run in to any more/ anything different on this subject. Thanks a million!