Within hours of Nullriver releasing its NetShare application through Apple's iTunes Store, Apple apparently put the kabosh on the application, which allows users to turn the iPhone into a modem. Users can, of course, still use the application on a jailbroken iPhone, if they can find it. Nullriver doesn't have the application available for download from its website.
Why bother? Though it could be argued that this may be AT&T protecting bandwidth on its new 3G network, I've got to think the prohibition is coming from Apple, as it had the same policy for the original iPhone. Given how easy it is to set up phones-as-modems on AT&T's slower network, and AT&T's acceptance of the practice, why not the iPhone/iPhone 3G, too?
Regardless of the reason, this sort of effort to control is simply going to drive more people to jailbreak their phones and buy fewer applications directly from Apple. When you can get many applications for free outside Apple's App Store, why bother paying?