If you don’t know by now, then now you know. Wearing your Gilneas city faction tabard while running normal or heroic level Wrath of the Lich King dungeons will earn you rep with the forthcoming faction. I went from neutral to friendly in two heroic dungeon runs. You will get at least one rep point for every mob kill. Remember, I said at least. Of course the stronger the mob the more it’s worth rep wise.
Just in case you don’t know where to get the Gilneas tabard, I suggest heading over to Darnassus. The quartermaster is on a horse at the near the flight master within Darnassus. He’s easy to miss because he’s in human form.
Side Note: Are the Night Elves the Welcome Committee for Azeroth? They welcomed the Draenei and now they take in the Worgen.