I'd put this down as strictly rumour at this point, but you can bet your movie ticket cash that if both Batman Begins and Superman Returns kick butt at the box office there'll be a good possibility that this comes together. BTW, that's the official Superman shield from the upcoming film, thanks to SuperHeroHype.com.
From Garth's excellent Dark Horizons site:
Frequent Flyer' claims to have met the lovely Christian Bale at Heathrow last week and dropped in this: "He's still quite bulky from the Batman part! Anyway, I said Hi, got an autograph, wished him luck with Batman (asked me if I was going to see it - like, duh) and I said I hoped he's back for Batman again. "Two more actually, and then Brandon Routh, the new Superman, and I, are doing Batman vs. Superman as a movie".
Personally I'd love to see this (do have to add "done well"?), but it would be tricky since Batman and Superman will likely have diametrically opposed cinematic styles.
How do you mix dark and light without ending up with grey?