So now that metadata caching is out of scope here, we turn to frameworks, architectures, and application design. The issue Twitter has is in aggregation of their meta data. People who propose a distributed solution for Twitter obviously miss the inherent nature of Twitter; it's a centralized system. Twitter should remain as it is, but ditch RoR and become more modularized. This is where they start developing real systems; the type of systems they talk about in those mundane CS classes, like programming in C -- that stuff. Modularize the applicaion, develop systems that scream for aggregation, cache the hell out of everything, and start applying some computer science.
Twitter was born as one of those next-gen Web 2.0 "keep track of your friends" hip Ruby on Rails insertbuzzwordhere application. Now it poses actual architectural challenges. It's so similar to the evolution of Facebook. Just take a step back and look at it.