After getting a sneak peak of the iPad app store, I was disappointed to see more expensive versions of existing apps on the iPhone. I am a proud owner of "Flight Control," a piloting game that costs a buck for the iPhone/iPod Touch. The "HD" version apparently will cost $5! The game better bring some great additions on top of the real estate enhancements, or $5 is just crazy.
Other companies are and will be guilty of doing this. Just as the iPad offers greater screen size and performance, so should companies offer a greater experience, if they want to charge more. A bigger screen means you can do more at the same time, but if this "more" doesn't bring anything new to the table, a bigger price tag isn't justified.
We WILL notice a general increase in app prices because more apps will be fully featured, whereas a lot of existing apps are cut down due to limited screen space. I'm cool with paying more for apps that are worth my time, and more than just a simflified solution.
The iPad's 9.7 inch screen allows for more complex apps to be designed. Everything from multi-touch gestures to content discovery and interaction has to be adapted, which translates to new and unique experiences. Apple redesigned the iPhone native apps from scratch, with a completely different user experience in mind. Check out the Guided Tours to see what I'm talking about. Some 3rd party apps are already shaping up to provide awesome experiences.