Do you trust Amazon S3 or Mosso Cloudfiles not to lose or corrupt your data?
spkthed: Sure?  It's like anything else, don't trust a single solution.  In addition to local backups S3 is good, in addition to S3, Mosso.  There will always be accidents, data loss, corruption, etc.  The only way to mitigate that risk is simply to cover your bases and avoid relying on a single thing.