Here in New Zealand you can use the iPhone with a Vodafone Prepay Sim card, so technically there is no plan and no "monthly" bill as such. Of course, once you start using the data features it can get expensive, although they do also have a NZ$1 per day data "plan" and you only pay on days you use data.
I'm not sure if you can officially use the iPhone on Telecom NZ's Prepay Sim or not. Telecom NZ have just upgraded their network to proper 3G, so are currently offering people a NZ$600 credit to move or sign-up their iPhones.
Unfortunately both Vodafone and Telecom NZ's plan charges are ridiculously expensive and data capped.
There is a third network starting in August, but there's been no word on what plans, data caps, etc. they will be offering.
Hmm, seems to be some awfully mis-informed people around here.
Sure you can, it is called Pay as You Go (PAYG), you can buy the SIM from the provider and prepay for calls/texts, both AT&T and TMobile do this, AT&T also call this GoPhone.