I was just looking at info for Martin Campbell's previous films and discovered a couple interesting things I didn't know before....
I still think that highest chances are Campbell is working with James Horner on the music for Green Lantern, but there are two other things that got me just slightly questioning this a little bit:
On Campbell's latest feature, "Edge of Darkness", he worked with acclaimed composer Howard Shore for the music. Shore composed the music for each film in the "Lord of the Rings Trilogy", as well as films like "A History of Violence" (which was also based on a comic), and the critically acclaimed "The Departed".
And we all know that "Vertical Limit" was not Campbell's best or most succesful piece of work, however I just found that the score for that film was done by James Newton Howard. Howard is, as I'm sure you know, famous for the composing the duo scores for both "Batman Begins" and "The Dark Knight". He also composed for many other high profile action/fantasy films, like "I Am Legend" (also based on a comic book), "King Kong", "Peter Pan" (2005 Live Action), as well as the upcoming films "The Last Airbender" and "Salt".
Like I said, my bet is still on James Horner, but with this info that I just found out... I wouldn't be suprised if it was announced Shore or Newton Howard were the composer for GL. Especially Newtown Howard, with BB and TDK under his belt.