This is an important point and from a usability/efficiency point-of-view the iPhone apps will have problems in scaling up (even with increase resolution). Think about how inefficient a direct port of the weather app would be on the iPad. Also I believe the screen size is not proportional in the iPhone as compared with the iPad which will create additional porting problems.

This does represent an opportunity (or potential problem) as certain apps will need to be re-designed for use on the iPad. There is a potential for a number of apps to take over the market with reengineered designs made for the iPad.