It's worthwhile to note that Apple claims to machine the MacBook [Pro] cases out of a solid block of aluminum, and that they have a similar finish as the iPad on the inside. Of course, Apple may be taking some liberties with the truth, in that the "solid block" may start out as a die-cast part that is not too far from the final shape.
If you go here http://www.apple.com/MacBookpro/ and click on 'watch unibody video' you can see a lot of the construction process. It doesn't look at all like it starts as a die-cast part. However, I'd expect them to take whatever steps necessary to reduce the manufacturing costs of the iPad whilst keeping the build quality where they want it.
My guess is this is the machined out back as the rear of the iPad is curved.The inside doesn't have to look as pretty as the rear does so they probably only removed just enough material to fit the components and save on machining costs/time as it is not perfectly smooth.
This makes sense. I'm also wondering if this design adds any strength?