The exchange rate between USD and BTC is quite volatile. Either they have to constantly change the bitcoin price, or your pizzas will frequently get quite a bit cheaper or more expensive.
No need to speculate, they explain how this works right on the main page of http://pizzaforcoins.com :
"We set our exchange rate APPROXIMATELY $0.50 Cents less
then the current Mt.Gox Rate. The reason for this is that
with rapid fluctuation of rates and occasional drops, we
do not want to be left holding a worthless bag if the rate
drops for some reason. We update prices on a daily basis
to reflect this 50Cent rule."
I imagine stores counter that by setting the price in USD on the server side, and then using an API to convert it into bitcoin on each page view. Problem solved.
