For some reason I decided to see if iphone tethering would work if I was connected to wifi... it worked but not exactly how I was expecting. I think it was accessing data through the 3G not the wifi. I ended up turning off wifi on my iphone and it still worked!
It seems that Vodafone was listening to me moaning about how it's stupid how Prepay users can't access the www.iphone-vodafone.net.nz APN (therefore blocking tethering). So now I'm connected to the iphone specific APN and tethering is working.
So I have something good to say about Vodafone. Eeeww... it feels all strange inside

Can anyone else confirm that VF have enabled the www.iphone-vodafone.net.nz APN for prepay users?