How does the dropbox client for Linux work ?
Does it intercept read/write system calls using something like FUSE and then makes HTTP put/get calls to the dropbox server accordingly?
Linux supports file change detection through iNotify and so there is no need for any clever filing system hacks.
The really clever parts of any of the file synchronisation tools revolve around detecting which bits of a file have changed and only sending those parts (delta changes). Any file sync service that doesn't include this is pretty hopeless. Thankfully, Dropbox have this reasonably well nailed.