He also said something similar to the New York Times: "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."
I think it's interesting that Jobs is sort of blaming Cingular for the restriction (though his scenario is obviously grossly hyperbolic). Of course this simply carries over the iPod model to the iPhone, so I guess it would probably have been the case even if Cingular hadn't insisted. It doesn't mean, however, that we can't be disappointed.
[Via iLounge via Digg and Thanks to Matt for the NYT tip]