Over at iPhone Matters, Tanner Godarzi asks whether you'll be able to buy an iPhone without a new contract, speculating that stores with extra inventory may allow just that. I think that's pretty unlikely. The commercials clearly state "Use requires a minimum new 2-year activation plan". Godarzi also suggests that you might be able to add an iPhone to an existing AT&T contract, which sounds at least a bit more realistic to me.
Contract details, specifically the complete lack of them, continue to keep many possible customers away from making their purchasing decisions. How many people do you know who have said "I might get an iPhone--if it turns out I can afford it"? With rumors of $70 per month data plans, it could be a very expensive choice.