Completely unverified, but my money's on there is a server call when the play starts, which tells the server that this movie has begun playback and verifies that it has not exceeded 24 hours since the first call. Given that your local clock is 100% in your own control, that seems the only semi-comprehensive approach to this. If that is correct, then your movies are all still there, but unplayable.
On the other hand, if this is NOT correct, then it is quite likely that you would be able to rent a movie, play the first five minutes of it, wait a week, set your clock back to the time you stopped the movie previously, and play the rest. Which would be very nice, but a loophole studios would be clamoring to close the moment they caught wind of it.
Note that the iPhone sync screen does state that an internet connection is required to transfer any movie to the phone. I take this as evidence supporting my theory above