According to Gigaom, the e-commerce giant [Amazon] is working on a subscription ebook service called Kindle Unlimited, which would offer unlimited ebook rentals for $9.99 a month.
There is more here. According to one estimate it would be for 638k titles or so, of course it will matter a great deal which ones. I would consider this “developing,” but also “not yet confirmed.”
Addendum: Virginia Postrel offers a good analysis.