You didn't think it was going to be that easy, did you? The folks over at PS3 Fanboy are reporting that older PlayStation 3 and PlayStation Network games patched for support of the new trophy system, which goes live with firmware 2.40 this Wednesday, will not award trophies retroactively. The system will not scan older saves and award you trophies based on your progress. This means that you will essentially have to go back through and play the game all over again in order to earn your much-deserved Sony achievement points.
The reasoning behind this? I can go to several websites right now and download completed saves for any number of PS3 titles and pop them right onto my system, just like that. Suddenly everyone would have perfect award scores for everything, chaos would ensue, and eventually fans would burn Sony offices all over the world to the ground. Armageddon is not nearly as fun as it sounds.
Everything you need to know about firmware 2.40 [PS3 Fanboy via CVG]