Logically, wouldn't Apple save costs and improve manufacturing efficiency by nixing most of the parts used in the current iPod touch, and by instead relying the majority of the components and enclosures that the iPhone 4 employs? You need not order two sets of materials for similar devices. Granted, I may be off on this, but it makes sense on my head.
Therefore, I predict only minimal differences between the two devices, however a 5MP camera in the next generation the iPod touch still seems slightly debatable. Apple may think its music-centric users would prefer a thinner design and increased storage, rather than the implementation of a high-resolution camera. For me, they'd be wrong in that claim, but I digress.
That said, I see both the iPhone 4 and the 4th generation iPod touch being almost identical otherwise, retina display included.