The only difference we've seen in teardowns is a smaller Blu-ray drive (which still spins at 2x for BD, 8x for DVD and 24x for CD) and a rearrangement of internals -- both of which make way for the volume and size reduction.
Many had hope that eventually MS would come around and at least add native support for HD DVD, but considering the Blu-ray spec requires BD Java -- and we know how much MS loves Java -- MS insiders like Chris Lanier don't think it'll ever happen.