Since Madonna is something of a vampire herself, preventing aging through a combination of obsessive exercise, Malawi children, and God knows what else, it seems only fitting that she would somehow be involved with New Moon.
A source close to the production says there are no plans to cast the Material Woman in the movie, but she may provide some songs. Madge's manager, Guy Oseary, is a producer on New Moon, so it could all be pretty convenient.