Computerworld:
Other telcos, such as Telecom, 2degrees and Orcon, have indicated they too will be selling Micro SIMS that can be used in the iPhone 4 and iPad. However, none of these telcos have an official distribution deal with Apple.
“Nobody else has got a network that Apple trusts enough to run an iPhone on,” Brislen says.
With respect to Paul, but that's a big spin there... I'd believe it if it was coming from Apple.
I'd bet a reason for other networks to not sell the iPhone is volume. And it's not like the iPhones are locked.
[EDIT Included paragraph to add context]