I think that while carbonite may be good, something like JungleDisk, in conjunction with S3, may be better.
With S3 and Jungledisk you pay for what you use. Also, Jungledisk encrypts everything locally with a strong encryption, AES256, I think, before uploading it to the cloud.

Apparently, they added Rackspace storage and some point as well.

It is definitely worth a look.