I think Dropbox is approaching to the point when they have to build their own storage. You can't be a storage company without having your own storage solution in backend. It would be similar to Yahoo not having their own search engine while being a search company.
According to them, their storage costs are ~20 times less than those of S3. A bit different use-case for sure. But as wave says, once you become a storage company you will inevitably need to do it in-house to be cost-effective.
"You can't be a storage company without having your own storage solution in backend."
I am not sure that is necessarily true. There are plenty of web hosting resellers who are successful without ever buying a single server or a single line. They've repackaged and re-sold a service. Dropbox can operate in a similar fashion, re-packaging and reselling Amazon's services.
Yes, they can, and apparently have, but they're getting to the size now that they can achieve some significant economies of scale by building out their own storage infrastructure. Their profit margin has to be pretty low with S3.