Who's to say that the other DC superheroes won't be set in "the real world" after a fashion too, though? You're all speaking as though Batman from Batman Begins would have to stand shoulder-to-shoulder with the actual comic versions of Superman, Wonder Woman, Flash, and Green Lantern, when in fact they'd all likely be meeting somewhere in the middle. Unless the expectation is that the JL movie will be noticeably cartoonier than most of the big SH movies these days, I don't see why TDK's Batman couldn't co-exist with the movie versions of the other DC heroes. It seems like saying he couldn't exist in the same world as Iron Man or something like that (well, Marvel / DC universe separation aside).
To sum up: almost all the big superheroes get toned down to degree in their transition to film, in order to make them more believable in real life. Batman is a more extreme case, but that's mostly because he lends himself well to so-called "realism" since he has no super powers. Part of what makes the begins-verse seem realistic is that there are no super humans, but if you have Batman interacting with super humans in a JL movie, does that really require Batman's character to change, or does it simply mean that he is part of a bigger world than his standalone movies could show? I'd argue it's the latter.
Even if they tone the characters down, they're still going to be much more powerful than Batman. In Superman Returns, they powered Supes down, because they showed him struggling to slow down a plane's flight, yet he's still a hundred times more powerful than Batman is. If GL ends up being anything like the script indicates, GL is also going to be incredibly more powerful than Batman. In short, even if you tone them down, they're still going to be much more powerful than Bats is.
I keep seeing a lot of people cite Iron Man as being more "realistic" in terms of tone, but I thought it had much more in the realm of fantasy than BB or TDK. Tony is flying around in a high powered body suit that can blow up a tank, fly, take heavy artillery fire, and amplify his strength. That's much more fantastical than anything in TDK or BB.
Also, they havn't established Bats as having the tactical intelligence he does in the comics. At least there, he serves as a general of sorts, issuing orders and such. However, I still think it's rather silly when I see him in the midst of battle against superpowered beings. For example, in Green Lantern: Rebirth, Batman is at the fight where Parallax takes over the Guardian. What possible purpose can he serve there, other than issuing orders? He has nothing in his arsenal that could hurt Parallax, especially when the thing is in the Guardian.
What it comes down to is this: whatever the JL fight at the end of the movie, Batman really isn't going to have much to do. Because if there is a threat big enough to command the entire attention of the JL, Batman isn't going to be able to fight it. The only way I could see it working is if he's in a fighter jet of some sort, because I honestly don't see them giving him a superpowered suit if they want to keep it in the same realm as the BB and TDK Batman, since that world doesn't seem to have technology that advanced yet.