His major point is that with its huge install base (which topped 50 million iPhones and iPod touches months ago), it makes more financial sense to develop for Apple's platform, rather than Google's Android or Palm's webOS which might have on 5% to 10% as many devices on the market.
Giving developers an app store is the easy part. The hard part is bringing us enough customers. The iPhone is so good that it built up a huge installed base without any third-party apps, but no Android or webOS devices can say that yet.
Arment points out that the iPod touch makes a huge difference as well, giving developers a similar device to work on without the need for an expensive cell phone contract. He also echoes Fake Steve's comments on different hardware complicating development, though he thinks if Android popularity continues to grow, the platform might justify the investment one day.