A post to Apple's Web Page from Steve Jobs says it all:
"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.
"It will take until February to release an SDK because we're trying to do two diametrically opposed things at once?provide an advanced and open platform to developers while at the same time protect iPhone users from viruses, malware, privacy attacks, etc. This is no easy task. Some claim that viruses and malware are not a problem on mobile phones?this is simply not true. There have been serious viruses on other mobile phones already, including some that silently spread from phone to phone over the cell network. As our phones become more powerful, these malicious programs will become more dangerous. And since the iPhone is the most advanced phone ever, it will be a highly visible target."
"[...] The SDK will also allow developers to create applications for iPod touch."