The cloud computing market is changing. Previously, the big concerns have ranged from “Is the cloud transformation really happening?” to “Can I afford the cloud?” to “Why would I ever use the cloud?” But Cloud Expo 2012, taken as a snapshot of the industry halfway through the year, shows that the conversation is shifting, with most of the sessions tackling questions around “how,” not “why.” But the number one concern that seems to be generating buzz at Cloud Expo is around the question of openness and customer choice in the cloud.

This was probably best characterized by a keynote speech delivered by Henry Fastert, SHI’s Chief Technologist and Managing Partner of service provider SHI. Fastert talked about whether or not the cloud is ready for mission critical applications. Tellingly, his presentation touched on the idea that a mission-critical cloud needs higher levels of security, better connectivity and uptime, an enterprise-level facility underpinning it.

But more than that, a next-generation cloud service provider needs to offer enhanced customer choice, which includes more flexibility when it comes to providing public cloud (or “multi-tenant cloud,” as Fastert chooses to put it), dedicated private cloud infrastructure in a hosting facility, or managed behind-the-firewall cloud services. The idea also extends to leveraging open technologies like the Open Virtualization Format (OVF).

The benefits of those open standards, Fastert says, are twofold. First off, it ensures VM portability between private clouds and the public cloud, enabling the much-discussed hybrid cloud future, with VMs able to reside behind the firewall and burst workloads out to the public cloud. But having the VMs in an open format also means that the customer is able to take their images and move them around from provider to provider at whim. Open standards aren’t just a movement, reading between the lines. They’re the wave of the future.

Even Oracle’s Cloud Expo presentation hyped how open its new platform-as-a-service public cloud offering is (though the extent to which that’s true is debatable). A session by Rackspace Hosting touched on the idea that the public cloud isn’t right for everybody and tried to explain the reasoning behind different options for cloud hosting. And a talk by Rishidot Research Principal Analyst Krishnan Subramanian on the growth of open source and open standards (touching on OpenStack, CloudStack and other similar projects) in the cloud drew a huge audience.

Marten Mickos, CEO of open source private cloud developer Eucalyptus Systems took a different tack towards the same end in a presentation of his own. Mickos’ presentation, entitled “Clouds Are All About APIs,” put forward the idea that APIs are core to the very concept of the cloud. After all, Mickos says, the entire reason the cloud exists is to push user capabilities past what’s possible with the infrastructure they can touch.

In other words, an API economy is necessary to push the cloud forward. Eucalyptus, which recently partnered with Amazon Web Services for don’t-call-it-hybrid cloud services, has put its support behind the Amazon EC2 API, widely accepted as the standard for cloud infrastructure.

But generally and on balance, the cloud stack is no longer a stack, it’s a “symphony,” to use Mickos’ term. For any given piece of the cloud puzzle, there’s a service provider with an API to fill it in. APIs deliver on the promise of agility and competitive advantage, by enabling customers to choose how to extend their resources. To compete, developers have to offer an API. It’s that simple.

The final word (for now): The cloud is maturing, and with the first wave of cloud pretty well established, the eye turns to the future. And for all the proof you need that the future of cloud is open and flexible, take a look at the rebirth of Microsoft Windows Azure.