Codemasters is, sadly, the latest developer to be added to the list of those who won't be adding trophy support to their already released titles.
On the official GRID forums an administrator has posted saying that they're "currently not planning to restrospectively support PSN trophies - it;s not a trivial thing to add to a game" [sic].
Though this is disappointing, it's not too unexpected. Third party games are the least likely to be patched in. With Criterion showing their contemporaries, hopefully more will decide to rake in the goodwill of gamers by following suit.