A blog post from September 2010 by the Twitter engineering team, while published a year ago, describes very well the recent shifts in web app development. It goes into substantial detail regarding their 2010 redesign. As it turns out, the redesign was far more than a re-imagined UI, but in fact relied heavily on shifting the work done by the app mostly to the client.
The application also manages page state in the client, caching visited pages in the client while subscribing to updates and keeping the pages in sync with the state of the application.
The full article is available here.