Resume Information is a little bit more complicated than that. On HDMV titles, the Author decides whether Resume information will be stored or discarded for a certain title - you can only store the resume information for one title, so if you jump from the main feature into an extra, and resume information is held for the extra, it will overwrite the resume information for the main feature.
The PS3 in general is a funny beast, and I think you sometime have better luck hitting triangle on a video / dvd than x as this may give you options of "PLAY" or "PLAY FROM BEGINNING".
Where things get tricky with Blu-ray is on discs which are authored using BD-J which appears to be more and more titles - I believe if they wish to add some BD LIVE functionality.
By default, there is no resume information with BD-J as you are effectively running a programe on the disc, what is sometimes called a "Finite State Machine". When you stop the program that is it, the application stops.
What more and more BD-J developers are doing is 'capturing' the STOP command on your remote, and recording the disc's state to the player's persistent storage - Current Title, Time, Audio Track, Subtitle Track etc.
When you next press play, the disc is programmed to check if there is any information in persistent storage.
At least, I believe that is the theory!!