The short answer
It's still too early and there aren't any. People are starting to work on them though.
The long answer
Well defined and well accepted standards could make it possible to have things like interoperability and reliability and tell you what you can expect for functionality. They can also (in theory, anyway) help prevent vendor "lock-in". But they can also make life worse. Consider the myriad WS-* standards which are extremely complicated, suffer from feature creep, are hard to implement in products, and products are often hard to use. Many of the WS-* standards were defined by vendors without much input from the people who were expected to use them.
Are Amazon's API's the Answer?
Several vendors (Eucalyptus, cloud.com, Nimbula, and RackSpace to name a few), have implemented the same API's that are used in Amazon's EC2 and S3 for their own cloud computing environments. Marten Mickos, CEO of Eucalyptus, says "the Amazon Web Service API's are a candidate for a standard much like the PC standard of the 80's"
You can read about Amazon's APIs here.
Amazon is certainly the dominant vendor but the API is specific to Amazon's architecture and feature set. It is probably not complete enough to build into an industry standard.
Do Cloud Users Care About Standards?
Some do. Vivek Kundra, Federal Chief Information Officer of the United States, says standards are needed before the cloud can take hold in government. The federal government must develop standards so agencies don't duplicate inefficiencies in data centers.
Who Is Working On Cloud Standards?
A large group of companies has banded together to establish "a core set of principles to ensure that organizations will have freedom of choice, flexibility, and openness as they take advantage of cloud computing. While cloud computing has the potential to have a positive impact on organizations, there is also potential for lock-in and lost flexibility if appropriate open standards are not identified and adopted."
Other groups include the Cloud Security Alliance, Distributed Management Task force (DMTF), Europea Telecommunications Standards Institute (ETSI), National Institute of Standards and Technology (NIST), Open Grid Forum (OGF), Object Management Group (OMG), Open Cloud Consortium (OCC), and others. You can read about the various standardization efforts currently under way here.
Dissenting Opinions
Not everyone agrees that we need cloud computing standards. At a panel discussion entitled "Where Standards Are Going" at the Cloud Connect conference in March 2010, one member of the audience opined "Standards bodies are a solution in search of problem...the people on standards bodies are in danger of speaking in an echo chamber. After a while, all they can hear is themselves."
What do you think?
I leave you with a quote from Ron Knode, Director of Global Security Solutions for CSC:
"We will accept no cloud standard before its time.”