Sorry, I made a mistake in my previous post, the stop/play does actualy work on some movies and the PS3 with power off (again on some movies), so shift what I wrote by one click :)

though I did google (blu-ray "resume play") and found

[Link: forum.blu-ray.com]
[Link: hometheater.about.com]

but I don't know if it works and how.

The issue as I understand it is that BD content can be authored using BD-J or HDMV, HDMV is more advanced but similar to DVD and so resume works the same way, but BD-J is a programming language and some (not sure if all) have issues with resume (kind of like HD DVD where all the titles need to be HDi) because it needs to load the BD-J code before playing starts (and so needs to go back to the start)