Twitter is quietly testing an expandable timeline that allows users to see media and related information inside the timeline instead of off to the side.
The redesigned timeline, which was first reported by Patrick Bisch of Pinglio, allows users to click on individual tweets to expand conversations, media, and retweets. (See the video Bisch created below.)
Hovering over a tweet produces an "Open" link that allows users to open a tweet to see who retweeted the message, as well as view the entire threaded conversation. Opening a tweet with media such as a TwitPic image or YouTube video will expand the window in the timeline to view the media.