I guess it depends on whether Dropbox think they could implement the same storage service cheaper themselves. S3 actually offers a lot. They have at least four nines availability (and I think you can pay for more), a ridiculously high durability figure, plus geographic distribution and replication.

Developing that for yourself would be very expensive, I imagine. I'd actually be quite surprised if Dropbox went through the hassle of setting this all up. It would have to be a significant difference in cost to justify the effort.