[JockM]

As a result of an app I am writing, I can confirm that using the dropbox API is always accurate. This means you can set, test, and clear file locks using the API and be sure the effect (on the server) is immediate.

The API is very clear, and relatively simple to implement. Just RESTful calls and OAuth (SHA1 isn't even required, they support and encourage the use of PLAINTEXT). I will gladly offer any help (including working code to examine), for free if you desire it.

But the point is you now have a clear and definative way of using file locks with Dropbox.