There was a brief segment on The Dark is Rising on today's All Things Considered. It had a interview segment where Susan Cooper basically said that she had nothing to do with the script for the new movie and that the changes in the script she saw were too extreme (all in a very polite, understated way of course).
Here's a place where you can hear/read the segment online. Choice quote: "You do have to do violence to a book to make it into a screenplay - the two mediums are so different," Cooper says. "But the alteration is so enormous in this case. It is just different." Under the author's picture, you can also click on a link to hear her read a portion of the book.
Here's an essay from the same site from a book fan worried the movie will rob future readers from experiencing the original tone of the book.