You know why every single discussion about canon on the Internet (and elsewhere) is stupid?
Because nobody has once asked Christopher Nolan or Jon Favreau if they're "worried" or "concerned" about the fact that neither Iron Man nor The Dark Knight are canon. I, meanwhile, get that question all the time...
I'm constantly hearing from assorted keyboard monkeys talking about how they won't read tie-in fiction because it isn't canon, which is why they read the Star Wars novels -- which, by the by, aren't canon, either, and anyone who tells you they are is lying, and as evidence I point you to a) the history of the Fett family and b) the upcoming Clone Wars animated film, which I bet won't be consistent with the various CW novels and comics (not to mention the Genndy Tartovsky animated shorts) that are allegedly canonical -- and yet I bet not a single one of them has refused to see the most popular movie on the planet because it isn't canon.
And it isn't. The Dark Knight is based on the Batman comics that have been published over the years, but they make no effort to be consistent with them, and ignore them as they see fit. They're not canon. The "canon" of Batman consists of the various comic books published by DC Comics.
No, what we have here is the fact that, in this country, we view things on screen as more real than things in print. Part of it is simple numbers: more people watch TV and movies than read books and comic books. That's why when you adapt a novel into a movie, you've got an entire Academy Award category to yourself (and other adapters like you), but when you adapt a movie into a novel, you're a talentless hack who's just in it for the money (never mind that screenwriters are far better compensated for their work than prose writers).
The people who say they won't buy a Buffy novel or a Star Trek comic book because it's not canon are most likely lying, unless they have also refused to see The Hulk, Iron Man, The Dark Knight, any of the Harry Potter and Lord of the Rings films, etc. for the same reason. It's just a feeble justification for a prejudice against prose in general and against tie-in fiction in particular.
Keith R.A. DeCandido has written flipping great wodges of tie-in fiction, including thirty novels, seven novelizations, twelve novellas, twenty-one short stories, a bunch of comic books, and a partridge in a pear tree in the universes of Buffy the Vampire Slayer, Classic BattleTech, Command and Conquer, CSI, Doctor Who, Farscape, Gene Roddenberry's Andromeda, Marvel Comics, Resident Evil, Serenity, Star Trek (in all its incarnations, plus some new ones), StarCraft, Supernatural, World of Warcraft, Xena, and Young Hercules, and not a single frapping one of them is canon. So there. Find out more about Keith at his web site at DeCandido.net or his blog at kradical.livejournal.com.