I know I’m a little late to the game on this one. But I read about this recently in Modernist Cuisine (which I borrowed from the library after a several month long wait), where they mentioned that a baking stone made of steel would be much better at transferring heat to the pizza than a typical ceramic baking stone. Well, now they have it available for purchase at http://bakingsteel.wpengine.com/. And on Serious Eats, J. Kenji Lopez-Alt does a comparison of cast iron vs the baking steel. See the commenting for more discussion on the merits of the steels.
They have 3 different models, basically of different thicknesses – 1/4″, 3/8″ and 1/2″.
Unfortunately, I missed out on their Kickstarter campaign 😦 but I think i’ll be getting one (sooner or later) to see how it will work with our homemade pizzas. I usually make pizza about once or twice a month, and a good crust has been difficult to achieve. Then again, we don’t even having a baking stone – it’s just baked on a sheet pan – and I didn’t even know about the cast iron pizza pan.