Honestly, given that this is a CONCLUSION to Bruce's story, I think an ambiguous ending would have been somewhat of a cop out. I think fans and the GA had the right to know as to what happened to Bruce and not leave that up in the mystery as Nolan did with the ending for "Inception".
Or it could just be a case of clever editing where Nolan made the audiences think that Batman was going to die when he really didn't.
Plus, of all the people for Alfred to wish Bruce sitting next to..he just happened to imagine Selina Kyle sitting next to Bruce, along with her wearing the pearl necklace that was recorded to have not been accounted for?
Honestly, until Nolan himself confirms otherwise, the ending was proof enough that Bruce Wayne DID survive and didn't die in that explosion. I mean I'm all for respecting people's opinions, but I just don't get on how anyone else could think that Alfred seeing Bruce AND SELINA in the cafe was a figment of his imagination.
Plus....it was established in the prison arc, that Bruce needed to rediscover the will to live and the fear of dying in order to truly do on what many consider to be impossible...and I don't think it just stopped at him climbing over the wall to defeating Bane..I think that also applied to how the odds were stacked up against him when he was racing that bomb to the ocean....how, because of his new found desire to live..he was able to use that to succeed.
Plus...the whole point of the film was to make Bruce realize that he needed to live and have the will to live....to have him die after all that he just learned and went through would mean that he received no real development and that it was a cop out.
And frankly, I think that the only people who truly thinks that he died are the ones that wanted him to die to begin with.