Twitter's scaling needs are in the 11,000 requests/second and higher range (number from DHH). How is that working out, given that Twitter was developed in Ruby on Rails? If I can paraphrase the discussion so far:
Alex Payne (Twitter Developer): Ouch, scaling that big with Rails was hard, and we lost a lot of the elegance of ActiveRecord.
DHH: You could work with us to improve ActiveRecord to address this issue and continue to benefit from the elegance of the programming model.
People will draw their own conclusions from these perspectives. I personally live in the "make it work, then make it fast" camp. But the interesting point is not the discussion, it is the context: 11,000+ requests per second. Wow. 99% of all web application code will never need this scale. For most every project out there, advantage: Rails.