According to a post on the game's official forums, Crystal Dynamics originally desired to have Trophy support included in the shipped game. "The guys at Crystal have been looking into this for a long time because they really wanted to include it," a community rep wrote. "The good news is that I can confirm that a patch for the PS3 version of Tomb Raider: Underworld is being worked on that will enable Trophy Support."
We've contacted Eidos for comment on the patch and an expected release date. We'll update as soon as we receive word.