According to Ryan on E! News, Madonna will be involved in the sequel to "Twilight" which the second movie will be made on the second book of the "Twilight Saga", "New Moon". Guy Oseary is the executive producer to the movie starring Robert Pattison and Kristen Stewart.
It is unknown to whether Madonna will either act in the second installment to the saga film or sing for the soundtrack or both.
Gaycrest also said it's most likely she'll both appear in the film and do the soundtrack.