One of the easiest ways to create great looking UI’s in your apps is to use the same Photoshop and Illustrator assets that interactive design houses use to mock up an iPhone app:
http://www.teehanlax.com/blog/2009/06/18/iphone-gui-psd-30/
http://www.mercuryintermedia.com/blog/index.php/2009/03/iphone-ui-vector-elements
