According to this New York Times article Steve Jobs states :

The interesting tidbit is the very last sentence. That leaves a little opening that could mean outside developers working with Apple can get access. So it is definitely going to be locked down so that 'Joe Developer' can't just whip something up and stick it on there but that last line makes it sound like it will be possible for applications not written by Apple to get on the iPhone. One perfect example is the Google Maps widget.