Full disclosure: in my youth I read many, many Batman comic books. And yet, I have absolutely no recollection of this character. Now, I'm not quite willing to declare that she doesn't exist at all, because she might have shown up in some weird place I never read. But I read a lot of them and she made no impression, so my guess is she's made up.
Why? Why would they do this? Bruce Wayne is allowed to have weird sexual tension with women, it's true, but it really shouldn't be some stupid doe-eyed harmony-spouting weirdo. Also I question the choice to let her find out and then not make her move to Fiji. (You will recall that the old movies let the woman find out, but only pretty grudgingly, and then she disappeared.)
Also, apparently Kat(i)e Holmes doesn't want to be in the second one, or something. Now, for me, this would be a sign from above that Rachel Dawes is a stupid one-film character and life would be far worse if she were allowed to become a stultifying recurring one, but nobody asked me.
So they picked Maggie Gyllenhaal! Are they trying to make me hate this movie? Me, personally? Honestly!
Let's review. Rachel Dawes is a stupid hippie, and probably made up. Women in Batman films should go away because Bruce Wayne is not a maundering softie. Katie Holmes is weird-looking and Maggie Gyllenhaal is repulsive (and won't this destroy her indie cred or something?). We've already run one Batman franchise into the ground; now that we have a better one, can we at least attempt to make it work?