First we had Rackspace and NASA form OpenStack. Then we started to see companies like Cisco and Citrix join in. They were followed by companies like HP which made its entrance this summer.

And today marks the  most important news in months as AT&T has announced at its developer summit that it is the first major telecommunications provider to join OpenStack.

Here’s a statement from AT&T:

We’re housing our OpenStack capabilities on dedicated infrastructure in three AT&T data centers today, with locations in Dallas, San Diego and Secaucus, New Jersey. We plan to more than double the number of our centers with open-source capabilities in 2012.”

At its summit today, the company has made a big deal about becoming an open cloud provider. It launched a new app catalog that will support unwrapped HTML5.

Further, by joining OpenStack, it can  attract engineers that want to hack on the most challenging issues in technology. Hardware and software are coming together again as mobile devices have a deep requirement for interoperability. Then there is the challenge of developing networks for a new cloud environment. Virtualizing a network is the future and one of the most exciting frontiers in technology development. These are just tow of many new kinds of technologies that need to be developed for what is called a federated cloud.

OpenStack fits into what AT&T is calling its developer-centric cloud. In a blog post,  AT&T CTO John Donovan touts its “cost-efficient access to highly flexible, integrated computing and application development services. ”

Translated? Developers are the kingmakers. And AT&T has rightly recognized that success will come if it can build infrastructures that accommodate developers and the apps they build. Apps are scaling in numbers. Companies need an infrastructure that can handle that scale and the devices that people use to connect.