It's all but certain that the following actors have been hired to play the following characters in the upcoming Justice League movie:
28 year old Scott Porter as Superman
28 year old Adam Brody as The Flash
35 year old rapper Common as Green Lantern
21 year old Teresa Palmer as Talia Ghul (Daughter of Batman nemesis Ra's al Ghul)
Brody, Common and Palmer were among those who showed up for the casting call one month ago. I'm extremely relieved that they're going with actors closer to 30 than 20 in this thing. That's one point on the plus side for director George Millar.
I'm still ambivalent about this one, especially with it going into production during the writers' strike.
If you want more background on the actors and to see some cool actor/character images head on over to /Film.com where they've done a nice job photoshopping each actor next to the character they're playing (well, except for Green Lantern where they used an image of Hal Jordan instead of John Stewart).
Like I said, there's been no official announcement but the word is that the above info is pretty solid.