To make your web app respond nicely to the width of your iPhone set the viewport constraint in the <head> of your application:
<meta name="viewport" content="user-scalable=no, width=device-width, initial-scale=1.0, maximum-scale=1.0">
Use something like FastClick.
Keep your Javascript file tiny by using something like Zepto.JS because it doesn’t have all the cruft that goes into supporting legacy browsers.
Load images after the page has loaded and only once they are visible.
When you press a link, Safari applies the hover styling which looks weird when you eventually pop back to the page. Instead remove hover styles to improve the overall design.