There is a space on the mobile iOS experience that is so perfect for a button, yet that pattern is very rarely seen. Content is the new button and a prime piece of content on the mobile experience is the top header. I am glad to see that using this area to provide context and functionality is becoming more prevalent.
Google+ has utilized the space in the new app. The zooming scroll feature was delightful for a moment then quickly grew tiresome and gave me the woozles. But i applaud the utilization of the middle portion of the title bar.
Tweetbot introduced the first instance of this i’ve seen in the wild. One of the ux folks at my job actually tried to push this feature through a couple of years ago and i’ve been wondering ever since when it would take off.
I am a firm believer in content being the point of interaction, but i know it cannot be taken lightly. If the interactions are not discoverable or inconsistent the experience will suffer. It also needs to be implemented in the proper context – it doesn’t make sense all the time. But when it is done well, with subtle cues and quick response, it is the only way to go.