I recently wrote a piece for El Reg about why Java should be the "cloud operating system" and all research points to the fact that Sun has all the components to make it so.
In fact, Java answers many of the cloud questions: Java is open source, mature, standardized, has plenty of development and management tools, and APIs that support a vast array of other systems. Java also has a very large enterprise developer base, a key to getting new technology tested and adopted.
Of course, it's not totally clear what Sun plans to do, but I'm hoping that they become relevant in the cloud-scape as opposed to their current status of "phantom limb."
One of the big problems at the moment is there is no definition of what Cloud computing means to developers. People are often just replacing "internet" with "Cloud" which is not right...but then again, what is?
There will clearly be more than one Cloud vendor (in fact I met with 3 that are not Amazon, Google or another BigCo this week) and they will all likely be different. Until there are base definitions and standards in place it will be difficult for enterprises to move outside their borders.