I can only speak for myself, but I loved reading to begin with and getting free pizza was just icing on top of the cake. Growing up with immigrant parents, eating pizza was always a treat because we hardly ate "American food." :)
I know it is a touchy issue when it comes to whether it is the "right" thing to do to give external incentives for reading. The argument is that joy of reading should be enough of a motivator for kids to read and "bribing" kids with external rewards defeats the purpose of reading.
As a teacher, I remember receiving the Book It packet of information, but it's been awhile since I've received anything from Pizza Hut. I did a Google search and found out that it is still around. You can find more information about it here: http://www.bookitprogram.com
Anyone else remember the Book It program? Was is a positive experience for you too?