The IMDB's Eclipse info has already updated to include Kirsten Prout has been cast as Lucy:
According to BD Horror News, "Kyle XY" actress Kirsten Prout will play the part of vampire Lucy.
As you may recall, Lucy figures prominently into Jasper's (Jackson Rathbone) backstory, as she helped form a bloodsucker army along with the mood-manipulator's maker Maria and fellow vamp Nettie.
She was later killed by Jasper and Maria after turning on them. In the novel, she is described as tall, blond and pale, meaning the casting of Kirsten couldn't be more spot on.