> My other concern with this is that if the tarsnap server ever goes away, customers risk losing their data, since the server maintains the mapping of S3 objects to blobs. That's worrying. Assurances are not mechanisms.
Agreed. I use duplicity instead, which is a Free program that is similar to tarsnap. It backs up to S3, but uses many fewer PUTs (since archives are several megabytes).
Anyway, a full backup of my homedir (minus music) costs me about $1.40 a month to store (with incremental backups every night). A small price to pay knowing that if my laptop blows up, I can be right back where I started in just a few hours. (Or if I delete a file accidentally, it is back in seconds.)