I got a Twitter application update on my smart phone a few days ago for one of my accounts, and this morning I signed into that account on the website and found the update had also taken place there as well. Except for the sidebar having been moved to the left, I really like the new look. It's clean, crisp, and very user-friendly.
My guess is that the sidebar was moved to the left to make ad-space more appealing to potential advertisers, but I do not have confirmation on that.
The update is being rolled out incrementally, so if your account doesn't have it yet, you can preview it at fly.twitter.com. As you can see, universal icons have taken precedence over the text for the pages and links - making the whole site much less busy, allowing the eye to focus on the Tweets. There is also uniformity now between the website version of Twitter and the mobile version - which makes sense, because Twitter was developed primarily for mobile use.
Twitter has also ramped up the "networking" aspect of its social-networking site by making its suggestions for followers and topics much smarter in a "discover" field. It has capitalized on the user-created and user-developed #hashtag phenomenon, and integrated an easy-to-use, more relevant search by stories, activity, who to follow, find friends, and browse categories, complete with user-posted video and photos. It's quite brilliant.
Of all of the social networking site updates I've seen recently, this most recent one by Twitter is, in my opinion, the most innovative and helpful to the user base. Now, we'll see if my guess is correct about the advertising.