I think they'll use Barry Allen for several reasons:
. He's comming back in 2009.
. Geoff Johns might write the script, and he's a Barry fan.
. He's easier to "explain" than Wally or Bart, who's great part of their characterization is the fact they live under Barry Allen's shadow.
. He's better reconized by the audience because of the "The Flash" series with John Wesley Shipp.
And, i'm the comics, Barry is actually lightheart and funny. He's only more sarcastic and less wisecracking than Wally (That is actually serious and focused in the comics. They gave him Bart's personality on JLU.) or Bart. In Justice League: The new Frontier, he only appeared three times, and always in a tense situation:
In the first, his fiancée was in danger and he was really pissed of about that.
In the second, he did a quirky remark ("What's the matter, Grodd? Feeling a little starve for attention"?), but he was quickly attacked and was about to the held hostage by his own government.
In the third, the whole world was a few hours away from Doomsday by the hands of the Center.
In the comics, he's more light-hearted, but he does get's serious when he goes to action, but they can change that in a movie, so he's more "funny".
