Nope, the N900 Dropbox client, called DropN900, is a Python based app and is totally open. In fact, the original developer abandoned it when he moved on to other devices, and since then users have been improving the script. Check it out!

I use it several times a day to sync all kinds of data between my N900 and my Mac and other machines. It's very easy - just create symlinks in the Dropbox folder to any files or folders (yes! folders!) you like on the N900, and do the same on your machines you want to keep in sync, start the DropN900 app, and press the sync button. (It will also, separately, keep your photos and videos in sync.) I don't believe you can sync in this way with the N9 DropBox app, it seems to have been designed with a different sort of use in mind. DropN900 doesn't have a very intuitive user interface, but once you've figured it out, it's extremely functional.

I wish the same existed for the N9.