Being in that majority of people who haven't built Twitter-scale systems in the past, could someone explain to me why Twitter is moving to a new form of twitter ID in the first place? What is wrong with their current system?

Also, it seems strange that they would include the new ID in string AND integer form in their JSON. I realize that they don't want to break existing javascript apps, but isn't there a significant bandwidth cost in adding that sort of kludge to the API when you're serving a quadrillion of these api requests every day?