Apple announced and released a Software Development Kit for people who want to make their own programs for the iPhone and iPod Touch. This is great, because before this, to put unofficial applications on the iPhone you had to pretty majorly hack the thing to get access to it. Very annoying, and this should make the situation a lot better.
The SDK itself is free, but to sell/distribute the programs you make, you have to pay a $99 setup fee. After that, you can either sell your apps (with %70 of the sale price going to you, nice royalties) or freely distribute them from the iTunes App Store.
All of this stuff won’t actually be on the iPod Touch and iPhone until June, when iPhone Software 2.0 is released, but for now anyone is able to download the SDK and start making their own apps.