With JavaOne starting tomorrow I have been waiting to see what kind of interesting things Sun will be launching. If OpenSolaris + EC2 is the best they've got it may be a long week.
Some interesting vendors are running their software on the OS-EC2 platform. For example, GigaSpaces provides software for highly distributed, highly available environments and running it as a Cloud application is cool when you consider that most enterprises are not that keen on the fact that servers in the cloud are just part of the distributed network.
It also makes sense to run Zmanda in the Cloud, as there is really no reason why you shouldn't back up your databases online and offsite.
I guess the big question is if it matters that it's OpenSolaris underlying what is effectively semi-sophisticated web hosting?
My take is that it matters in the sense of Sun now has table stakes for Clound infrastructure, but that the operating system is not the important part. What matters is the ability to run a wide variety of applications with no necessary knowledge of what's underlying the apps or where they are physically located. Sun is definitely going in the right direction but this isn't earth-shattering just yet.