From Neven Mrgan’s tumbl:

In landscape orientation, anything between 4:3 and 2.39:1 feels natural to humans. Now switch to portrait – 16:9 feels pretty weird. […] Every aspect ratio is a compromise. If a device is ever to be used in portrait mode – and my guess is that people will use the iPad in this book-like mode most of the time – that compromise must result in something closer to 4:3.

Neven has published an image of what the iPad would look like in 16:9. I can’t imagine Apple designing anything that looks this silly.