Jobs didn't say "It runs Mac software" because he doesn't want to scare non-Mac people away. He didn't use the word "Mac" for a reason. Non-Mac people get very nervous if you tell them they will have to use a Mac. Most of the world doesn't use a Mac, but anyone is a potential customer for the iPhone, just like the iPod market. Instead, they are emphasizing the main focus, the main reason to buy the thing: it's an iPod and a phone, and it will work that way no matter what PC you have. You don't need any other reason to buy it. If it runs Mac software or 3rd party apps, that's a plus. Instead, he said:
"It runs OS X" (with a huge Mac OS X logo! filling the screen)
"It uses Cocoa, Core Animation, etc."
"It has widgets"
"It runs desktop quality apps"
These were code-words to the loyal followers. We know what they mean. Windows users don't, and that's okay. Don't you get it? Jobs would not have used the term Widgets if it wasn't going to run Mac Widgets. That would be silly. And, Jobs wouldn't have mentioned Cocoa if developers couldn't use it. Cocoa and Core Animation are developer technologies. If developers couldn't use it, he would have just talked about the great features the iPhone has and left it at that. For iPods, you never heard him talk about the development features. We don't know how they develop iPod software, because it doesn't matter.
This tells me, it will run Mac software, and developers will be able to use Cocoa and Core Animation, and everything else they already use on OS X to develop new software. Maybe there will be additional APIs, or settings in Interface Builder. But, it won't be a completely different API, it will be Cocoa, that's what he said.
As was mentioned, developers have been told to get ready for "Resolution Independence" in Leopard for coming "high resolution displays". Gee, what new displays do we have that are high res? Only the iPhone. It's 160dpi.
Maybe it will be some time before developers can write software for it so that Apple can fully control the direction and future of the device before they have to worry about breaking 3rd party software - I don't know. But, it will happen. We have (select) 3rd parties developing games for iPod now. Some day, it will probably be for any developer. But, the iPhone will probably allow 3rd party software early on.
The cool part is, after millions of people buy the iPhone, then Apple can inform them that they already own a Mac and can run Mac software. They are Mac users! All of the sudden the Mac doesn't seem so scary and weird. Now they can switch painlessly. And, if they really need to run Windows, their Intel Mac can do that too. No worries.