I noted the polarizing effect on the wifi only iPad in the Apple store a few weeks ago. I was wearing polarized sunglasses and could only see the screen in landscape.
Yesterday I bought a 64GB 3G, and fitted it with a Speck anti glare screen protector before turning it on. Upon turning it on, the screen is not polarized in any direction.
Dunno if that's because of the screen protector or if the 3G glass is somehow different. Wouldn't be the first time. The 24" iMac screen and the 27" screen have different polarization.
Light polarization is very sensitive. If you pass it through certain materials, the effects of polarizing the light are lost, as it becomes unpolarized again. This is why 3D movies in theaters require specific materials to be used for the screen itself, to preserve the polarity of light reflecting off of it.
All LCDs use polarizing filters (both the old style used in digital watches, and what you see on your LCD TV or computer display). It's part of the core design of the LCD. The light from the backlight goes through one filter, then the crystal matrix, and then another filter at a right angle to the first. The electrical signals to the sub-pixels control how much the light 'rotates' to match the front filter. Done this way, you turn a pixel off by just making it so that the light doesn't rotate, and is instead blocked by the front filter. (Wiki has a more in-depth explanation on this)
Interestingly enough, this is the same reason you don't get real deep blacks without also controlling the backlight like current LED TVs now do. Two polarizing filters at right angles won't block 100% of the light passing through, just the vast, vast majority of it.
What's likely happening is that the anti-glare film has a property to it that will scramble the rotation of the light coming out, unpolarizing it. It might be a side effect of the design (it is meant to provide a matte surface to light, which has lots of small imperfections at specific angles), or intentional. Not sure.