...So! The new guy in charge of Warner Bros. said that he wants a "Justice League" movie, surprising nobody, and that the studio firmly intends to keep making "Batman" movies even after the current Christopher Nolan-helmed version of the franchise ends next year - ALSO not a huge surprise.
Everyone knew that Team nolan was zealously opposed to broader DC-Universe weirdness seeping into their hyper-realistic vision of Batman (it's widely believed that Nolan's extreme displeasure at having "another" Batman conflicting with "his" helped scuttle George Miller's in-production "JLA: Mortal" movie back 2007); but everyone also knew that Warners/DC was going to be champing at the bit to pull the trigger on "Justice League" as Marvel Studios "Avengers" got closer to reality; and with "Green Lantern" and a new "Superman" both on the way people were connecting the logical dots...
...and then Zack Snyder, director of the soon-to-lens "Superman" movie, "clarified" that despite the timing, HIS "Man of Steel" was still being set up as it's own seperate thing a'la Nolan's Batman - if Superman appears in "League," he'll be a different guy. Confused? Let me throw some
Here's the #1, all-important, explains-it-all Rosetta Stone for understanding EVERY SINGLE DAMN THING you will hear or read the studio, filmmakers and P.R. people say about "Superman" from now until the day it comes out: WARNER BROS. IS ONLY MAKING *THIS* MOVIE BECAUSE THEY HAVE TO.
That's not to say that they don't want to make Superman movies EVER - though it's been obvious for more than a little while that the folks in charge of the WB greenlight don't really have a burning desire to be in the superhero business - but the reason they're making this one right now, with a script that was still being finished at the time it was announced (and is allegedly still being "worked on) and with a presuambly expensive fanboy-proof-shield in the form of Christopher Nolan's "producer" credit is because they are staring down the barrel of a gun: If they don't make a "Superman" movie NOW, they risk losing a shit-ton of money in the near future. (This is also probably why Snyder, well-regarded among the Warner suits for his skill at bringing genre films under budget and on an efficient schedule, got the offer.)
Short Version: For the last few DECADES, Warners/DC has been in a legendarily-contentious ongoing legal battle with lawyers representing the estates of Seigel and Schuster, the original creators of Superman, over the rights to both the character himself and also various portions of the mythos (re: Lois Lane, Daily Planet, Krypton, etc.) Though Warners has recently set up a legal stalling-tactic, right now they're playing defense: Back in 2009, it was determined that WB had to get a Superman movie into production by this year (2011) or they'd owe subsantial fees to the creators' estates - hence their urgent desire to get this thing going. And even then, in 2013 they stand to lose between half and ALL rights to the character, and will have to renegotiate the whole bloody thing.
In other words, they're rushing to make "Superman" because they have no choice, and they're not going to tie it in to anything else because it's still "up in the air" as to how much (if any) of him they'll still be able to use afterwards. Notice you aren't hearing about any of the actors signing on for the new movie singing any contracts for sequels, for example...
Meanwhile, have you noticed that NO ONE on either the fan or filmmaker side is even raising the issue of the "Green Lantern" movie? Granted, that's mostly an indicator of how far the lackluster trailer and promotion has blown it off the pop-culture radar... but y'know what's interesting? "Lantern" was supposed to be WB's first toe-dip into continuity-land back before they (briefly, it would seem) backed-off from that course: A cameo by a major non-GL DC character got cut from the script just prior to shooting.