Via Millarworld, here's an interesting bit from Wizard's "Best of 2007" issue. Grant Morrison has a couple things to say about the upcoming end of his "All-Star Superman" arc.
MORRISON: ...But there will be some more after [issue 12]. We’re planning to do some All-Star specials with other artists.
WIZARD: Are the specials going to fit into the continuity of the previous 12 issues?
MORRISON: Yeah. It’ll be the world of All-Star Superman, but you might get to see what it was like in the first year of his career or an adventure with the Legion of Super-Heroes—that kind of thing. There were a bunch of stories that I had left over because I knew we were only going to do 12. I had an idea for an All-Star Superman/Batman book, and a bunch of ideas for that ended up feeding into the notion of doing these specials."