I remember Chinook running ads a few months ago that were boasting having the iPhone on their network legitimately, due to the fact that AT&T didn't have service in this area. They eventually dropped the campaign (likely because it was complete rubbish). To clarify, you can use the iPhone on any GSM network, and there's no special deal between Apple/AT&T and Chinook. In the end, you have to unlock your phone and void the warranty just as with any other non-AT&T GSM carrier.
One thing I'd like to point out is that T-Mobile has perfect coverage here, and unlike AT&T, they won't give you the boot if you roam for 100% of your minutes. There are quite a few benefits to using T-Mobile as well- lower data rates for one ($19.99 for unlimited EDGE and HotSpots), and free roaming. Chinook's main fault is that they are so local that roaming really adds up, unless you live 100% in Montana/Wyoming. Now, this may have changed with the Cellular One changeover, I'm not sure.
The only reason I'd sign up with Chinook would be if you're a fan of supporting local economy. However, now that they are essentially a Cellular One branch, I'm pretty sure that's gone out the window.
I've used AT&T, Chinook, Alltell, and T-Mobile here in Montana, and I can say with total certainty that T-Mobile is the best solution for using an iPhone.