This is one of the advantages of coming in to a new field second. Microsoft entered the achievement field and the first thing that happened with the system was that certain people exploited the system and swapped saves to fly up the leaderboards and get points they didn't deserve.
Sony, having witnessed the madness that surrounded the whole "gamesave" debacle and have decided to make the PS3 saves not work retroactively. PS3Fanboy confirmed this earlier on, as well as the Official Playstation Blog. This was to the delight of new Trophy hunters but to the dismay of those who were hoping to load up some old saves for easy Trophies. Looks like you'll be playing all those games again.
Wow, this could actually be a level playing field throughout! We're still praying for the inclusion of some sort of leaderboard. Come on Sony, pull through for us!