Characters/pairing: Bruce, Gordon, Harvey, Bruce/Harvey if you're so inclined.
Spoilers: I hope you've seen the movie.
Notes: As novin_ha
He doesn't tell Gordon, and hopes that the man doesn't notice the faint pulse. It's one of the many secrets he keeps from the closest thing the Batman has to a friend (Bruce Wayne has friends and acquaintances, but he hadn't really been Bruce Wayne for a while now). Gordon doesn't need another secret to carry after all, the weight of the existing ones is going to slow him down enough. Convenient, that, considering he'll be the one heading the pursuit.
Bringing someone back from the edge of death isn't easy, and isn't cheap, but Batman knows which doctors in the city can be bought, and Bruce Wayne has money to buy both the services and the silence. Bruce Wayne can also bring Harvey Dent to his apartment for the time of reconvalescence, something Batman could never do. Bruce was Rachel's childhood friend, could act in her memory.
Batman can be blamed for Rachel's death, and for anything and everything, but Bruce Wayne can sit at Harvey's bedside and look him in the eye and offer comfort that will be accepted.
Sometimes it is useful to wear a mask.