I was concerned that they would need to license the different formats separately, thereby generating contention for scarce resources, but that appears not to be the case. This is really great!
I hope Overdrive will have a suitable web interface so that Kindle owners (K2 on) can access the library portal efficiently and download directly to their Kindles using the (still experimental) browser. I doubt Amazon is going to add a 'borrow this book for free' button to the Kindle Store pages.

'
In answer to a previous question, Adobe does make money from fulfilling ebook transactions with Adobe Content Server (the backend of all Adobe DRM ebook storefronts and of Overdrive ePub/PDF borrowing). Not sure if the same rate schedules apply to both. I assume Amazon makes money on Mobipocket library borrowing (since they'd need to supply the device authorization from their servers) and assume the same will be true of Kindle format borrowing.
So: Overdrive pays the publisher for royalties on sales; they pay Adobe and Amazon for licensing their DRM.