I saw this article today.
http://www.buildingwebapps.com/articles/13
The exec summary: Sure Ruby on Rails scales and so do most frameworks if you use them correctly. But, scaling sites is hard work and so many people forget that it has a HUGE amount to do with the actual code and how that code uses the database (or not).
Over the weekend I also ran across this article of a Rails based facebook application that is pretty much exploding at the seams.