OK, I'll bite.
Rails scales perfectly well if you know how to write apps that scale. I know of several websites that handle millions of hits per day (try the Yellow Pages website- recently reimplemented in Rails with no issues whatsoever).
Twitter's picked on because actually it's not brilliantly well-written, and wasn't managed particularly well. See the example of the cdbaby guy who gave up because he didn't know how to write Ruby and just went back to PHP which was heralded as a 'why Ruby sucks' advert..
Rails as a framework is eminently scalable. If you think otherwise, you're eminently stupid.