[flyt]

Take a look at nCrypted Cloud for an option: https://www.encryptedcloud.com/

Realistically, though, Dropbox can't offer many of the features that differentiate them in the marketplace (collaboration, previews, sharing, etc) if all of the files they store are opaque to them. Playing in the "generic blob storage" market alongside S3 is a losing game for pretty much everybody.

[incongruity]

AFAIK, right now, there's no convenient S3 desktop client that makes it trivial to automatically mirror and share folders, much less make client-side encryption of files trivial on top of all of the above. IMHO, that's a very viable niche... User experience matters – it's not solely about cost.

Also, about n-crypted cloud – last time I looked, the file size limit was 150 MB – that kills the utility for my team, but it's still a decent option from my experience...