The problem with trying to streamline their inventory this way is that the iPad uses the same rear camera as the iPod touch, and the touch is too thin to fit the iPhone's camera module.
If you haven't noticed the iPod Touch is comatose right now. It got practically no update last cycle ( a white case only). The other issue is that that camera sucks in both the Touch and the iPad.
if the iPad 2 moves down into the $299 ($300) range that would wipe out the price range for the Touch (currently $199/$299/$399). Either the touch would have to move down (and wipe out or push down the nano ) or it would disappear.
If the iPad 2 only moves to $399 (and touch shifts down $50 across the line) it would not be all that hard to slightly increase the Touch thickness as to not to decapitate the camera. Recycle an iPhone 4 like case for Touch. It wouldn't kill them. The Touch is gratuitously thin; as if it suffered from anorexia .