I'm of the opinion that he's dead. If not, then that'd mean that Gordon and Wayne are both abominably unobservant, since both examined Two-Face's body. Besides, how would he get away? So badly hurt that two experts think he's dead, and the entire area surrounded by police officers and dogs who'd be poring over the scene in absolute detail. Unless his various injuries and damaged mental state gave him super-powers or something trashy like that, he couldn't simply spring to his feet and bound away.
If he's 'brought back' in future I'll accept it, but for me it really doesn't wash with a lot of what seems logical.
Besides, it'd be a little cheese-flavoured to have both the Joker and Two-Face survive after being flung from tall buildings.
I think this film exhausted a lot of the potential of Dent's character. We're showing why he's a hero, and what makes him 'turn'. He's got no reason to go and become a super-villain; he's just a man who was passionate and charismatic, who got driven beyond desperation and salvation by losing everything he cared about. There's not really anyone left for him to take revenge on, except Batman himself, and that's not a fight he could win - or even start, convincingly - on his own.
Or so I see it, anyway.