The Vodafone NZ website clearly states that the iPhone (3G) will not be “available” on Prepay. For various reasons, I swapped the SIMs in my prepay Vodafone phone and our on-contract 3G iPhone last night. The iPhone worked without any problems at all with the Prepay SIM.
Phone calls and SMS worked fine, as did (as you would expect) all the non-phone features of the phone. I didn’t sync the phone, so it’s possible it’s disabled there, but that would be easy enough to work around if you had two SIMs as we do (and I doubt it actually is disabled there anyway). I didn’t try getting data (no plans are available for any Prepay customer, but the casual data is available) since we’re in a non-3G area, but I expect that also works.
Possibly by “available”, they mean “available to buy” – i.e. unless you have a contact with them they won’t sell you one. However, we bought ours outright and they didn’t even get my phone number IIRC, so they had no idea if I had a contract or not.
I guess they want to sell more contracts, but being honest wouldn’t really hurt that much, would it? They might even sell more phones, although maybe they don’t make much from that (I’m much more likely to get one if I can stick with my Prepay access).