On 16:9, Windows 8 tablets are not an eReaders.
To those arguing about how bad or good is the 16:9 aspect ratio on a tablet you need to consider this, Windows 8 tablets are not designed to be eReaders.
The 16:9 aspect ratio on a tablet is as odd as playing games, watching photos, video, or putting side by side apps in a tablet, all things you do as often if not more than reading books or magazines.
It is incredible that as the world progresses to wide screen there are people that still defend the 4:3 aspect ratio on tablets while at the same time these devices evolve into multimedia consumption, gaming and productivity devices.
The 4:3 aspect ratio has been abandoned from PC and has been widely adopted for a reason, the 16:9 is much more versatile.
And the portrait mode thing, thats just an urban myth... the main argument I hear from people who say you can't do portrait mode on 16:9 screen is that it looks odd, let me tell you that if you are going to dismiss something because it looks odd, you may have other problems to fix first. One thing I particularly like about the portrait mode is the on screen keyboard, that suddenly allows me to type fast and accurately, while at the same time, I'm able to see a lot more content.
So, I think Apple will migrate to 16:9/10 at some point in the future, more so if they want to allow more than one app to be seen at the same time without going back to the Windowed environment. 4:3 is not suitable for side by side apps viewing because you will need to deal with a layout that is too narrow and too long and that can be a big challenge for app development. While with Windows 8 apps support 2 modes natively and they are designed to work as a side app or as a main stage app and only the app you choose to work as a side app will suffer layout changes. They have started already with the iPhone as always, they may bring some innovations to the space like weight balancing or altering the shape of the device or probably going to an in between aspect ratio, but if they don't want to be left behind as with the big screen race, they will have to act.