One of the great things about the iPhone is how limited it is; to design an application for it, you just need to take one very simple idea and implement it really nicely. With the iPad, it won't be as simple; you have a lot more screen real-estate and the interaction model with the device is totally different. Can you imagine a fart app selling on this device? I certainly can't.
Instead of scaling up iPhone applications for the iPad, it's going to be more that we'll have to scale down desktop applications. There's an instant disadvantage to any developer who hasn't got established desktop software in their repertoire. Most of our iPhone apps simply will not work on the iPad because the apps' concepts do not fit this new platform. Noel of SnappyTouch has already 'announced' that he will not be porting his existing iPhone app to iPad, and I think that will ring true with many other developers.
The sky is the limit with the iPad platform, and there are some exciting times ahead. As mentioned in the keynote, this will be a whole new gold rush for developers.