I always thought it was quite obvious, but maybe I'm wrong.
"You either die a hero, or live long enough to see yourself become the villain. I can do both those things." Bats is simulataneously being a hero (through his becoming the villain) and living (through the death of his Batman career). With his lie, he has secured peace for Gotham.
How does any of the suggest he was throwing in the cowl?
For goodness sake, in the trailer they mention peace time.
8 years later. They also mention the Mayor is going to get rid of Gordon because of it. If Gotham was in peace time for 8 years then why wasn't Gordon dumped by the Mayor a long time ago if Gordon's surplus to requirements during this blessed peace time?
Dent's actions have inspired good, and when everyone doesn't tolerate the mob or internal corruption, Batman is no longer needed. This hypothesis is further strengthened through Gordon, explaining to his son, that Gotham deserves Batman, but no longer needs him.
No, Gordon said Batman is the hero Gotham deserves but not the one it needs right now so they'll hunt him, because he can take it, because he's a silent guardian, a watchful protector, a Dark Knight.
None of that suggests Batman is going anywhere to me. That says to me that Batman is going to willingly drop the image of being a hero and be the outcast and bad guy and let Dent look like the good guy for the sake of Gotham's hope.
Retirement or quitting doesn't come into it, IMO.
The question then becomes, why does Batman return? Its simple. The storm is coming. The fire is rising. A new breed of evil is being born, and Bane seeks to destroy all the hard work and sacrifice Batman and Gordon did, and make Harvey Dent an idealist of the past. Batman msut come back, Gordon says. I don't know why all the fan boys seem to disregard what I think is simple analysis of dialogue they complain is too explanatory.
Maybe because not everyone perceives this information the same way you do, and it's not as straight forward as you personally think it is. Gordon says they were in it together, and then he was gone. Now this evil rises. Why would Gordon be throwing a relationship they had 8 long years ago in Batman's face now, unless they'd still been working together watching over Gotham in those subsequent 8 years since TDK?
We see Bruce is hobbling around with a cane in the trailer. Doesn't that also suggest he's put out of commission first before Bane makes a bid for Gotham? Wouldn't it be more strategically sound to eliminate the only real opposition before taking over Gotham, like he did in Knightfall where he broke Batman before he took over Gotham?