The Apple iPhone is not a true smart phone. Like the iPod, the iPhone is a closed platform. You can buy content such as music and TV shows, but not third party applications.
Apple will be the only party to provide applications for the iPhone according to CEO Steve Jobs. The only real way to provide third party applications will be through the safari web browser.
I have owned several smart phones, and the feature I like most is the ability to buy and download applications. I really hope Apple opens this up in the future.