Is there a Dropbox app in the store?

As far as I'm aware the answer is no. There are apps that will let Apple TV play video content stored in shared folders that are visible on your network. Plex & VLC for example. These could potentially draw from a Dropbox folder on a computer.

tt2

doubt there will be one

pretty sure the sandbox environment would block the dropbox app from launching or even knowing of the existence of the various apps which would be used to access the various files stored on the dropbox drive