First they cast Christian Bale, who is neck and neck with Robert Downey Jr. for the guy everyone wants to cast right now. With Bale on board to play John Connor, it seemed that this was on its way to becoming the best Terminator movie yet. [Or at least not a train wreck. - Zach]
Then they cast Josh Brolin, a fairly hot commodity himself since No Country for Old Men, to fill the Governator's shoes as the new Terminator. Then Charlotte Gainsbourg was out and Bryce Dallas Howard was in.
And now they've managed to get Bonham Carter, who even further adds to the sense that this is becoming more of a "film" than a "movie." Which should be tough to do with McG directing. But with this cast, I doubt even McG can make it sound like a brainless blockbuster.