Smart move by The Economist. They're working to leverage the blogosphere by providing early access to content. The goal is to generate buzz about upcoming content before it appears in their magazine or on their website.
Smart, but tricky. The key to success here will be to see whether they can truly engage the blogosphere rather than appearing to simply use it. That probably means dedicating resources to work with each blogger and not just doing mass e-mailings of pre-pub information every month. Will they make it easy for the bloggers to interact directly with writers, editors and others associated with each article? This will be crucial if they expect the bloggers to spark discussion about this material. Few of these high profile bloggers will be patient enough to wait for answers back from a "PR spokesperson" or anyone other than the person who wrote or edited the material; they'll expect the same sort of transparency from The Economist that they're trying to promote through their blogs.
Again, provided The Economist is approaching it with the appropriate level of attention and investment, I think it's a fantastic idea and I look forward to reading more about the results in the future.
Book authors: There's a tip in this for you as well. It's always a great idea to see if you can leverage the blogoshpere for your upcoming book. Work with your editor/publisher to see whether they'll help you (or at least support you!) by providing final, edited chapters or other excerpts for this sort of thing.