I think if the iPhone comes with the same price plans as today you'll see Sprint sell a boatload of iPhones.
Remember, though, that the smartphone market is far from saturated. You could argue that it's barely been tapped - dumbphones still compose the vast majority of the U.S. phone share. Upgrading from a 3 year old dumbphone is a prime opportunity for a consumer to rethink their phone carrier. I imagine Sprint lost a lot of first time smartphone buyers to the iPhone thanks to Apple's "easy-to-use" reputation and the massive hype of every iPhone debut.
I have a hunch that this will apply to the iPhone too. Of course, you would be able to jailbreak then tether, but I'm not sure how many ordinary users would be willing to do that.
According to my first reference below, tethering will also be capped at five gigabytes with the launch of the iPhone. So, you'll have to pay $30 for tethering five gigabytes a month. The internet on the handset will still be unlimited.
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1. http://www.dslreports.com/shownews/Sprint-Caps-Official-Teth...
The only benefit for the locked ones are that you get to buy it on what is essentially cheap credit, but it's not like you save any significant amount of money on that.