If you listen to geeks, locking out development of third-party applications will doom the iPhone in the market. But remember the now-famous review when the iPod was released:
No wireless. Less space than a nomad. Lame.The market quickly decided they didn't care about wireless and bought the things in droves. And current versions have more space than the nomad did when the iPod came out. Now that the iPhone has been shown, geeks are again claiming that it's going to fail. This time because it's not going to be open to third-party applications.
Apple doesn't care if you can extend it because they believe their target customer doesn't want it extended. They want something that works well, the same way, every time. The iPod wins because it does pretty much what people want, close enough to how they want, without making them think about how to do it.
The iPhone may not be open to developers, but it's upgradable. When Apple finishes writing software to make the Wi-Fi automatically pick up a hotspot and act as a VoIP phone, that functionality can be rolled out transparently. First-gen iPhones will become second-gen iPhones without the users having to do anything.
The upgrade path will be to higher HD capacity, so people can carry more movies with them. I see these things as hugely popular for people who take trains to work. If I could take a train where I work now, I'd already be on a waiting list for an iPhone.