This is awesome stuff. It really adds a lot of end-user benefits when your app is able to sync via Dropbox.

While the APIs are only for iOS and Android, on BlackBerry 10, it's technically baked into the OS. If the user has linked their device to Dropbox, the OS automatically syncs the filelist and presents it as a part of the filesystem. This means that the user can open, edit, and save files directly to their Dropbox, without having to worry about connectivity. This also means that every BlackBerry 10 app can access your Dropbox folder, for free.

Even still, I have a feeling the Sync API likely offers something more than what I've described, but I haven't had a chance to delve into it. But I'm glad to see Dropbox branching out like this.