"The feature we are beta testing is called 'Contributors' – it enables users to engage in more authentic conversations with businesses by allowing those organizations to manage multiple contributors to their account. The feature appends the contributor's username to the tweet byline, making the business to consumer communication more personal; e.g. if @Twitter invites @Biz to tweet on its behalf, then a tweet from @Twitter would include @Biz in the byline so that users know more about the real people behind organizations."
They go on to note that this feature is one of several in development, and some features will be visible to regular users, and some will not.
This particular feature sounds like a business blog that can have many writers, so the ability to tweet is gaining a feature similar to corporate blogging ... the lines get blurred a little bit.
