Jobs said in an interview that the iPhone is currently a closed system:

“You don’t want your phone to be an open platform,” meaning that anyone can write applications for it and potentially gum up the provider’s network, says Jobs. “You need it to work when you need it to work. Cingular doesn’t want to see their West Coast network go down because some application messed up.”

Makes sense, unfortunately. Still, I get the impression that selected applications may apply for inclusion providing Apple can verify them thoroughly. Hm.

Update: In another interview (registration required) he confirmed what I said above:

“You don’t want your phone to be like a PC. The last thing you want is to have loaded three apps on your phone and then you go to make a call and it doesn’t work anymore. These are more like iPods than they are like computers…

These are devices that need to work, and you can’t do that if you load any software on them,” he said. “That doesn’t mean there’s not going to be software to buy that you can load on them coming from us. It doesn’t mean we have to write it all, but it means it has to be more of a controlled environment.”

This definitely mean’s there’ll be some sort of certification or verification program for selected third-party developers.

Let’s hope Apple has a non-cellphone, “OS X”-based tablet somewhere in the pipeline; that could be used as a development environment without spooking the cellphone providers, and selected/certified apps could then migrate to the iPhone…