I have converted an iPhone application using the wizard like thing in XCode into a universal app.
It builds fine but obviously looks a bit rubbish in some areas :)<
Actually, Apple does all this automatically, just name your NIB files:
MyViewController~iphone.xib // iPhone
MyViewController~ipad.xib // iPad
and load your view controller with the smallest amount of code:
[[MyViewController alloc] initWithNibName:nil bundle:nil]; // Apple will take care of everything