Hmm... never noticed that.
I'm not sure if that would happen though. Despite mentions of some involved saying they're keeping the door open for a third movie, Nolan has stated before that he prefers to focus on the movie at hand and I believe will tie off all the major loose ends. Lack of closure, however interesting, can be a turn-off for quite a few people as there's that empty feeling.
Take the Sopranos for a minute. A few of my friends were big fans of the show, and were absolutely livid that was how it ended.
One even wanted to throw his DVDs of the show in the trash. So it may spark some decent water cooler discussion, I doubt many would be satisfied with such an ending. Also, given that a Nolan-directed third movie isn't guaranteed at this point (but a third movie made seems to make sense, and will more than likely happen) it's very possible that his story may get lost in a conversion. Similar to X-Men: The Last Stand. But this is all speculation.
Myself, I would prefer some kind of finality to the character. Something that places him in a specific point by movie's end. It's not like The Joker hasn't busted out of Arkham about a thousand and one times, so at the very least, placing him in some kind of custody at the end would suffice. Would I enjoy the film if that wasn't what we got? Sure, but if left openly ambiguous as to his whereabouts, I'd feel a little disappointed. But certainly not devastated to the point where I'd trash my DVD and comic collection over.