I actually do this all the time when we order pizza: simply tear the box in half, divide the pizza in two, and eat it right off the cardboard. (It helps that I'm a voracious eater and can consume, at minimum, half a pizza in a sitting.) Green Box pizza box is a far more elegant solution that serves four and doesn't require you consume a whole pizza ...
While it's not yet available at your local pizza joint, Green Box (designed by Eco Incorporated) has several patents pending for this design that saves paper plates (or the washing of your dinner plates). Wouldn't it be great if all pizza stores began selling pizza in these recyclable boxes?
In the meantime, my advice to you when you order pizza is to get out the scissors and turn that box into a nice set of recycled cardboard plates.