I just listened to a great podcast from the Gillmor Gang. Staring front and center was Blaine Cooke, Twitter's former Chief Software Architect. Some interesting discussion surrounding Twitters scaling issues were highlighted, plus some future feature talk regarding topics such as grouping and tracking.
It was interesting hearing Blaine break down some of the issues that his team endured during Twitter's early days. I'm a strong believer in Blaine's obvious technical ability, and I'm even more impressed after his admission that Twitter had only three engineers, including himself, up until September/October of last year. It helps to put into perspective some of the issues Twitter was having when you realize the limited human resources they had at their disposal.
This podcast is definitely worth a listen if you're in any way interested in Twitter's history and/or software architecture.