@jack is back @twitter

Twitter co-founder Jack Dorsey today confirmed rampant speculation that he is returning to the micro-messaging service. Not surprisingly, he did so via a tweet:

Today I'm thrilled to get back to work at @Twitter leading product as Executive Chairman. And yes: leading @Square forevermore as CEO. #200%

The executive chairman post is particularly notable for what it isn't: Chief executive, a role currently held by ex-Feedburner boss Dick Costolo. Some speculation had been that Dorsey would push Costolo into some other position, although it never was really explained what that position would be.

Also, the reiteration of Dorsey's commitment to Square must soothe some nerves among the VCs who have plugged over $37 million into the mobile payment startup. Those firms include First Round Capital, Kleiner Perkins, Khosla Ventures and Sequoia Capital.

Twitter said the following in an official statement:

Twitter is pleased to announce that Jack Dorsey has agreed to return to Twitter in an everyday role to lead our product development. Jack will also continue in his role as CEO at Square.

Twitter's three co-founders --Ev Williams, Biz Stone and Jack-- have unselfishly played whatever role was most needed at the time to nurture the company and help the product reach its full potential. Jack has been involved with the company from day one in various roles.

As executive chairman, Jack will dive in to work with more than 450 people, led by an experienced executive team. The timing is fortuitous; not only is Twitter experiencing record growth, but we also now have a new infrastructure that will keep us ahead of that growth and enable us to launch products that will make Twitter more instant, simple and always present.