Written by Brad Meltzer and drawn by various artists, the issue presented the reader with vignettes set "yesterday" and "tomorrow." The final vignette was set "today" – i.e. the formation of the then current JLoA roster.
The "yesterday" segments incorporated various bits of JLA lore and brought them back into continuity.
The "tomorrow" segments presented bits that may (some [one] did) occur in the stories that followed in the DCU.
The final "tomorrow" vignette, drawn by Jim Lee, has now become the most significant to me.
In this vignette, Superman, Batman and Wonder Woman discover the appearance a second Earth. The third panel on the page is a gallery of various JSA and JLA members in their "classic" attire and the fourth panel shows the two Earths with slightly different geography.
Superman and Batman discuss:
Clark: How's it even possible?
Bruce: It's vibrating at a different speed.
Clark: No, Bruce....I mean, how can it even be?
Bruce: Clark, we've seen alternate universes, pocket prison dimensions and a full-on multiverse. Why should we suddenly be surprised to find....that there's a second Earth out there?
Could this page have been a foreshadowing of the DCnU?
Jim Lee is one of the architects behind the formation and "bible" of the DCnU and he's also the artist on the new JUSTICE LEAGUE series.
Dan DiDio has stated that the Justice Society of America is being given a "rest."
In the DCnU's sliding timeline, the emergence of the super-hero began five years ago (which will be chronicled in JL and ACTION COMICS [and possibly in other DCnU books, i.e. BATGIRL]), which seemingly puts the kibosh on the original JSA roster, that team having formed during WWII.
It's possible the DCnU JSA kept their activities on the down low during their reign in the 40s and 50s, doing what they do best – being heroes – but not drawing media attention to themselves (remaining "urban legends"), being true "Mystery Men" and "Mystery Women."
But after reading JLoA 0 again, I think one of the stories we're going to see – set at some point in the sliding five years past – is going to be the Justice League discovering a second Earth. A second Earth, where the JSA is still doing what they do best – being heroes and training their next generation.
I think "SECOND EARTH" will be an arc in JL and it will lead into a full exploration of the "second Earth" in MULTIVERSITY next summer.
There, I said it!
8.27.2011 UPDATE:
Fan Expo Canada: James Robinson will be writing a DCnU JSA series set on Earth-2. 
