Have you noticed how regular web pages look so small on the iPhone? By default the Safari browser shows you the whole web page which is supposed to be a LOT bigger than the iPhone display. But some sites have great 'mobile' versions of their site that fit and behave perfectly on the iPhone.
Now instead of learning all the javascript and browser detection scripting required to do that, you can just use psd2css Online and have your iPhone web app design coded and working in seconds.