you can use the screensize and/or screenresolution to make inferences about the device.
Maybe my English is not so good but I guess you mean that I can get information about the current device's screenresolution and make the app adapt accordingly? If so, that might be useful but not really my question.
I have managed to create one UI for the iPhone and another for the iPad in AS3 with CONFIG::-code. So I can create two diffrent swf-files and then of course create two different ipa's. But if I want to create a "universal" app (Apples term for one ipa that works differently on iPhone and iPad) I need something more to make the two UI's end up in the same ipa. Something that makes the device interpret the CONFIG-code (maybe then in some other form) and display the app depending on the device. What could that be?