Great news! Straight from the horses mouth, (well, Steve’s anyway) Apple have announced that an iPhone SDK will be coming in February.
Let me just say it: We want native third party applications on the iPhone, and we plan to have an SDK in developers' hands in February. We are excited about creating a vibrant third party developer community around the iPhone and enabling hundreds of new applications for our users. With our revolutionary multi-touch interface, powerful hardware and advanced software architecture, we believe we have created the best mobile platform ever for developers.
Update: if that dissapears (as there seems to be no permalink for this article), have a read of Mac Rumors copy of the whole article.