If you're developing a web application or site in anticipation of the iPhone's arrival, you can either adjust a Safari window to be 320x480, or you can download the free app iPhoney and give it a try in a more aesthetically pleasing context.
iPhoney has an embedded WebKit view, so the behavior should be more or less accurate, although it doesn't yet support pinching and zooming.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070620211735966