There's a thread on this over at the bay12 forums, this post is by the author.
"This book represents an experiment by O'Reilly in "real time" publishing. This means the book is likely to get frequent updates - I believe that people who buy the book will actually get updates pushed to them as the book is maintained on an ongoing basis by me (and possibly others). I love being part of an experiment, makes me feel dwarfy."
I'm guessing that's in reference to an e-book version as I can't see them making money mailing out new pages and chapters as they're done.