Twitter as a scalability case study

A lot has been said already about Twitter's scalability issues. Many have given Twitter as an anti-pattern of how not to deal with scalability and have suggested different solutions for scaling it. As Twitter is famously a Ruby-on-Rails deployment, this case has also been used as a weapon in the language/platform wars between the RoR and Java camps, and to a lesser degree, also with the LAMP (PHP) camp