Joss Whedon wrote the introduction to the graphic novel Identity Crisis published by DC Comics, which includes the following snippet:
Mainstream comics (particularly the two titans, DC and that other one) are a universe, one that is as established and often unfathomable as the real one.
Besides writing for X-Men, a Marvel keystone, he’s now set to direct The Avengers movie, which is surely Marvel’s most ambitious film to date. Has he even written for DC?
As for Identity Crisis, it was good, not great. I enjoyed the overall mystery, but the story jumped around a bit too much and included a lot of two bit villains I didn’t know a damn thing about. Don’t get me wrong, I always enjoy learning more about lesser known characters (I keep my phone handy to check out their wikipedia pages) but I felt a few too many were introduced and never really contributed much to the overall story. Still a fun read though.