I can't see Microsoft including a blu ray drive, given their behavior this generation. They're definitely focused on digital distribution. By 2015 blu-ray playback won't even be a selling feature, because by that point anyone will be able to go down to their local walmart and pick up one for $50. I don't even care about my consoles having dvd playback nowadays, hell I must have about 6 devices in my home capable of DVD playback. Blu-ray was much more of a talking point back when these systems launched, I suspect as we go further into the digital content realm that any talk of bluray playback in a future xbox will largely be met with indifference. By 2015, blu-ray will be a 9 year old technology, about the same age dvd was when Blu-ray came into the marketplace.
I'd sooner put money on MS, if they continue with physical game distribution, to create their own disc tech specifically for xbox 720, before putting in a BR drive. And I would also fully expect, that digital versions of all games will be available day and date with physical games. The best approach in terms of gaming content delivery is to appease both customers, those who want hard copies and those who just want their games stored on a hard drive.
Too many systems and games....not enough time or money!