A friend of mine told me that there is a new rule to pass the appstore approval process. The app also have to work on iPad otherwise it would get rejected. I didn't find something which supports his statement. Does somebody know what is true?
Yes, it is part of the new App Store guidelines posted by Apple.
Section 2.10 says: iPhone apps must also run on iPad without modification, at iPhone resolution, and at 2X iPhone 3GS resolution.
You normally need a dev account to read these guidelines, but in the meantime they have been mirrored by other people. Google for "app store guidelines" and you'll find the document.
no, the applications can be installed or not depending on the device, but they all are in the app store. For example, if I'm using an ipod and try to install an ipad application it will tell me that it's incompatible and viceversa