It's practical ergonomic design. The chamfer allows the curved underside to have a much larger radius. This makes it easier to pick up the iPad from a flat/slippy surface by 'hooking' underneath with your finger. I don't believe the half circle iPhone profile would allow you to get enough of your finger 'hooked' underneath. You just end up pushing the iPad.
The half circle profile shape works with the iPhone because it's small and light enough to grip from both sides with one hand and simply pick it up.
That said, I would also like to see the industrial design of the iPad to evolve to something softer at the edges.