It is an open source research project, not an actual product offering at this time. This means Sun's services are offered for free. Of course, there is one catch with regards to the Sun offering: you must have a Grid account, and you will be charged for resources used on that grid.
The source code for the entire stack is freely available today. Not just the programming APIs, as in Google's case, but the entire stack. If you are comfortable using Glassfish, Postgres, and "limiting" languages to Java, Perl, Python, Ruby, and PHP, you can start your own Caroline-compatible cloud computing company today. Just remember, its a research project, so all of this is subject to change.
In some ways, this is what you would expect from Sun: an engineering research project touted as the future of computing. No charge for the software, etc, but note that Sun can actually monitize this through the Grid-hosted offering.
I haven't played with Caroline yet, but I think Sun is at least marketing the platform I hoped that Google, or Microsoft, or Adobe, or someone out there would have built. Yeah, its Sun, so its probably a computer science dissertation project to configure and manage the thing, but who else is doing five languages on industry standard infrastructure with RDBMS support?
I'm hoping to get around to evaluating this in some detail in the next couple of weeks, so stay tuned.