Check out this demo video below showing the polarization of the iPad versus the iPhone.
DisplayMates Ray Soneira give a nice explaination to ZDNet
Using polarized sunglasses all iPads go black in Portrait mode. Other displays go black in Landscape mode. Much better is for the manufacturer to set the extinction at 45 degrees so the display looks good in both Portrait and Landscape modes. The Motorola Xoom behaves this way, with compensating films this effect can go away almost entirely.
The iPhone 4 and Samsung Galaxy Tab have no extinction at any angle (just a small color shift). The effect should only apply to LCDs because they use polarized light internally. So OLEDs also should not show any such extinction effect.