Okay, I'm writing this because all the theories on Batman's death seem to forget the Bat-Signal being restored. It's not a figment of Gordon's imagination, he physically reached out and touches it. It's doubtful that Alfred is the one to replace it and since Blake is just discovering the batcave at that time there's no way its him.
Given Bat's showcased knack for slipping out even when some(usually not so many, granted) are watching him makes the idea that he couldn't escape "Because no one saw him" very irritating. Though tough, it wouldn't be impossible for the master of misdirection to accomplish.
To those who think his proximity would make it impossible for him not to be irradiated, well lets face it. If that were true, all of Gotham would be impacted by the fallout. He couldn't have gotten too far out, so its more than likely that whatever fusion based enery was used in the reactor though stated as nuclear, is unlikely. Possibly hydrogen based or similar is more likely the reactors true nature if they weren't evacutating the city right away.
Alfred is clearly not faking his reaction to Bruce's death so the end scene with Selina Kyle couldn't be part of his fantasy since she's wearing the necklace sitting across from Bruce Wayne. Again, they state the necklace as missing, unneededly stated unless for setting that scene up.
Ah the autopilot...this was an unnecessary hint if he was dead. He not only blatantly lies once, but twice that the autopilot was nonfunctional. Clearly he wanted to implant the thought into Lucious and then for Gordon most likely. The editing of the last moments leave the time frame nice and open, they never show a wide shot of Bat's inside the cockpit to probably muddle your thoughts on it. Batman probably escaped around the time the explosion hit the building, Nolan cleverly makes it seem like the cockpit scene could only be after he's well over the water. In any case, by the building or over the water, he likely engages the autopilot and escaped.
With all that said, I again bring up the Bat-Signal, if Batman died and Alfred is unaware that Blake is to be the new Batman, who remains to repair the signal. It's unlikely that he went back to Gotham with a city-anihilating bomb on the lose, but stops off to replace the signal before even grabbing his suit. So, timeline wise it could only be after things calmed down.
Bottom line is Batman has to be alive...though I'm open to the fantasy theory, the evidence points in the alive direction...sorry if this ruins your thoughts but come on, he's freaking Batman. If he can take on Superman, a little bomb is nothing.