According to this New York Times article (Phone Shows Apple's Impact on Consumer Products), the new iPhone will not allow 3rd-party developers to write their own software for it. Yes, the iPhone runs OS X, but nobody outside of Apple will be able to write applications, extensions, add-ons, widgets, or anything else that you can normally do with OS X.

Is this a huge problem? Probably not, especially if Apple's bundled applications meet users' needs. But I was personally very excited that non-Apple developers would be able to write little applications for the phone. Over the years, many little utilities, features, and extensions that are part of OS X (or previously Systems 7, 8 and 9) started their lives as a small project written by one person, and over time it caught Apple's attention enough to pull it into the OS. That's not gonna happen with the iPhone. And I know it's not a huge problem, maybe more of a minor bummer?

“We define everything that is on the phone,” he [Steve Jobs] said. “You don’t want your phone to be like a PC. The last thing you want is to have loaded three apps on your phone and then you go to make a call and it doesn’t work anymore. These are more like iPods than they are like computers.”