An interview with one of the Twitter developers.
Once you hit a certain threshold of traffic, either you need to strip out all the costly neat stuff that Rails does for you (RJS, ActiveRecord, ActiveSupport, etc.) or move the slow parts of your application out of Rails, or both.
If you're looking to deploy a big web application and you're language-agnostic, realize that the same operation in Ruby will take less time in Python. All of us working on Twitter are big Ruby fans, but I think it's worth being frank that this isnt one of those relativistic language issues. Ruby is slow.
Scaling anything is hard and this seems to be a common payoff, you can get to a good size quickly, but scaling past that is hard - or slow to get there and scale well. I would always, always rather be faced with the former problem.