(Some new links and stuff have been added on the bottom.)
The short version: Amazon pulled all of Macmillan's titles (print and otherwise) from their website due, apparently, to the publisher's desire to raise ebook prices. Speculation says this is because Amazon wants people to buy the Kindle, making higher prices for ebooks bad news when it's a big publisher pushing the price game. But there are also speculations that this is in response to Macmillan's deals with Apple, about which Amazon is not at all happy.
The long version: I'll send you to others for that, because I don't want to simply repeat what has already been said. There's Scalzi's three posts on the subject, then Writtenwyrdd's take, and Tobias S. Buckell's take.
My take: I can't help feeling like this is going to end up poorly for Amazon. Yes, playing hardball seems like it would work in principle, but I don't think Amazon has seriously considered the competition coming their way via the Nook, Apple's iPad (a minor nuisance right now), and the dozens of other companies pushing the ePub format. It could go bad for the publishers too, I suppose, but Amazon is the one that really should be rethinking its business model right now. Because if the Nook and the other ePub format folks take off, then Amazon will be unable to compete or negotiate because the publishers will no longer want to work with them on electronic format (or print format even, since B&N does both, and generally at the same prices as Amazon). Amazon will have to consider pushing other media over books.
But what do you think?
(An even more detailed analysis of what is going on can be found here at Tobias S. Buckell's blog and at Jay Lake's blog. You can read Macmillan's response here.)