During the interview with Walt Mossberg at the D Conference, Apple CEO Steve Jobs hinted that support for third-party apps on the iPhone will be available eventually after dealing with the security issues an open-platform might carry, the last thing Jobs wants is an iPhone that can be easily hacked or that crashes a few times a day.

"I think sometime later this year we will find a way to let third parties write apps and still preserve security. But until we can find that way, we canít compromise the security of the phone. Nobodyís perfect, but we sure donít want our phone to crash. We would like to solve this problem, if you could be just a little more patient with us, I think everyone can get what they want." Jobs said.
Jobs also confirmed the iPhone will indeed be running a full version of OS X with a different user interface tweaked to work with a multi-touch screen. It is only a few more weeks until the launch of the anticipated iPhone in late June, only then all the speculation surrounding it will finally come to an end. You can read Steve Jobs interview at the AllThingsD website.