S3's durability numbers are for a single region (since they were quoting that number before they supported cross-region replication) -- so you'd get much better durability if you mirror your objects across regions. Which, apparently, is what Dropbox does.

Though if you require such high durability, you're better off with multiple providers in multiple countries, since at 11 9's, it probably becomes more likely that a provider will go out of business or a civil disturbance will make your data become unavailable than the chance of them losing it.