Unfortunately, Amazon's S3 isn't really a great backup system. Its service agreement does not make any guarantees your data will actually be there when you want it.
The service agreement specifies 99.9% uptime, with service credits being paid if they fail to meet that mark: "AWS will use commercially reasonable efforts to make Amazon S3 available with a Monthly Uptime Percentage (defined below) of at least 99.9% during any monthly billing cycle (the âoeService Commitmentâ). In the event Amazon S3 does not meet the Service Commitment, you will be eligible to receive a Service Credit as described below."
It's not perfect, but I'm not aware of any companies offering any better uptime guarantees
Of course, it isn't that good example of a cloud in the first place. Amazon is still the sole provider, making it just another remote storage company.
I think you're confusing cloud computing with a distributed p2p network. I'm not knocking p2p, but the major players in cloud computing are pretty well accepted to be Amazon, Google, Microsoft, and Salesforce.com: http://en.wikipedia.org/wiki/Cloud_computing