A message from Steve Jobs just got published on the Apple site announcing an iPhone SDK for third party applications on the iPhone.

Apple plans to release the iPhone SDK in February of 2008. Apple needs that time to develop an open platform for developers that is also safe for the iPhone customers. Meaning Apple does not want to open the door for viruses, malware, privacy attacks, etc.. Steve Jobs highlights Nokia's move to digital signed code on mobile phones. Digital signed code is something Microsoft has been doing for a very long time.
This should make many iPhone power users happy. It is still a couple of months wait, but this could become an important factor for the success of the iPhone.
Read the Steve Jobs message on the Apple site. Via digg.