It is if you're storing JSON blobs for contacts and simply send it up to a cloud storage service. It would be last-to-write wins, in this case. Supposedly, Dropbox would handle this sort of scenario.
Of course, this is assuming you weren't using a database (even a NoSQL DB) and simply updating the changed fields, which is probably how you'd realistically handle this situation.
EDIT:
Taking a closer look, it seems like the Datastore API is more of a way to synchronize multiple databases/stores (in essence). One stored locally on a phone (that can be changed, in their example, in airplane mode), one in the cloud, and one on the desktop, or anywhere else your app runs with the Datastore integration. While there are relatively simple ways to locally queue changes made while offline, and then sync them up to a master database in the cloud when the network comes back, Dropbox's Datastore seems to do this for you without you having to worry about it.
Of course, this is assuming you weren't using a database (even a NoSQL DB) and simply updating the changed fields, which is probably how you'd realistically handle this situation.
EDIT:
Taking a closer look, it seems like the Datastore API is more of a way to synchronize multiple databases/stores (in essence). One stored locally on a phone (that can be changed, in their example, in airplane mode), one in the cloud, and one on the desktop, or anywhere else your app runs with the Datastore integration. While there are relatively simple ways to locally queue changes made while offline, and then sync them up to a master database in the cloud when the network comes back, Dropbox's Datastore seems to do this for you without you having to worry about it.