Not only is it not a surprise but I think Amazon.com are also well aware that Dropbox is likely developing their own storage back-end to move away from S3.
It makes sense. Their biggest running cost right now will be Amazon. Up to a certain size S3 makes sense as it would cost more to run the infrastructure yourself and it might be less flexible (scale wise) as a smaller operation can't afford to have an awful lot of spare capacity lying around. As the userbase continues to grow though, there will come a time when running their own infrastructure will work out cheaper and more flexible and they might be able to profit from it the same way Amazon do (remember: Amazon's "cloud" services came about from wanting a way to make profitable (or at least cost neutral) use of all their spare capacity in the quieter ten months of the year), so they would be daft not to at least have some embryonic plans simmering away on their R&D people's back burners.
Migration costs and effort will be high. Still, they need to become less dependent from S3. We opted for dedicated hardware at https://secure.cloudsafe.com and do not regret it: Higher security level, more uptime and in the long run much cheaper. Even if you pay some extra dollars for redundancy with added storage capacities.
