Looking at the reply to a message a game developer from Burnout: Paradise. He states the trophies are retroactive. However, it seems possible this is only for the online aspect of the game.
This looks like a great way to play.
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it keeps getting better
I put big time into this game. This is really reassuring. I've been itchin to play this again anyway. WOoooT.
Link directly to the comment in question > http://blog.us.playstation.... (#39)
It sounds as though retroactive Trophies will be the developers call. It might me some extra work who knows, but the guys at CRITERION have it coming...
"* Trophies will be retroactive, so, if you’ve achieved something today, the day the update arrives (not Cagney, we want to emphasise) that will be reflected in your Trophies."
Im so happy! this should deffinaly happen with warhawk as well. hopefully COD4
He also mentions that the Burnout Paradise gamesaves cannot be copied. Looks like they had this planned since day 1. It's impossible to copy a gamesave for this game because it's locked to your console like he said so no wonder why they're retroactive for this game.
I think it'll be the same with Warhawk. You can't copy a Warhawk gamesave therefore the trophies for that game will probably be retroactive as well.