For those who don't recall, Pullman is also the author of the atheistic "His Dark Materials" trilogy (written for children as an anti-Narnia), the first of which ("The Golden Compass") was made into a movie.
Carl Olson has blogged about some of the plot off the book (even though he hasn't read it yet):
It just came out on April 1st, which was both "April Fool's Day" and Holy Thursday (i.e. the start of the Sacred Triduum, the high point of the Christian Year). I can't imagine that not being a deliberate choice for a release date.
Basically, it's a novel about how Mary had twins: the humble, good, somewhat dim-witted "Jesus", and the cunning, not-so-good "Christ". "Christ" conspires to have "Jesus" killed so he can start a new religion.
Before we get too worked up about it, note that it is currently ranked #44,903 in books on Amazon.com. Even the "old news" Da Vinci Code is ranked considerably higher (#7,733). And Scott Hahn's The Lamb's Supper beats them both (#3,865 :)). Thus, the book does not seem destined to make many waves at this point.
It seems like Pullman is trying to pull a Dan Brown and cash in with a "provocative" work that "reimagines" Christianity. I doubt he will succeed. His children's books weren't nearly as successful as their counterparts (Narnia and Harry Potter). He just doesn't have the audience. Which is good news for us. :) But, I like to be aware of these things just in case I encounter it.