We’ve just been told that Trophies will not be awarded retrospectively, you’ll need to start the game again to obtain any Trophies that game may offer. This is a bit of a shame, but with the ease of save game sharing on the PS3, it’s only fair.
In addition, smaller PSN titles won’t offer platinum Trophies. Treat this as a rumour for now until we hear anything official.
Via email, via PS3Fanboy.