Twitter for iOS has been updated to version 5.7 with a new compose window that makes it easier to share photos and preview how tweets will look before you send them out. The overall timeline design of the app has changed, geting rid of those 'gutters' that many users didn't see a reason for, this gives all the pages a slightly wider view.
While the description doesn't say so, the new compose and wider timeline only seem to apply to the iPhone version of the app. You can now also receive a notification when a friend has joined Twitter and various improvements and bug fixes are in this update.
You can grab the update over on the iOS App Store.