Trusting online services is an interesting conundrum. Whenever we use an online service, even something as common as Gmail, we believe that the company offering the service is going to do what they say they do – keep our information private. So, we regularly transfer sensitive information over channels that are somewhat insecure, yet fundamentally trusted.
The same is very true for backups. In general, you are safe when you trust a company like Carbonite or any of the other major online backup services.
Can the people at that service access your data? Theoretically, yes.