The author of this article sounds like a caricature -- it's hard to believe that he's serious, but he is!
If not for Mongrel, he might be right. If you needed a separate copy of Apache running for each instance of your Rails app, I would agree that you would have trouble. :) I am interested in how painful it is to scale Rails. Twitter is by far the largest deployment and we all know they had some growing pains. I'm a big fan of Ruby, and I'm using it in a side project that I hope to turn into a startup, but I'm concerned that I might run into trouble in the long-term. (I'm a .NET developer by day, but I'm not willing to commit to Windows hosting, and I'm not too confident in mod_mono either...)
I'm not sure what this guy is talking about, and I don't think he is either