iPhone Web Clip Icon has been widely publicized, so there’s nothing really new here. This post basically is for my personal reference.
http://developer.apple.com/iphone/devcenter/designingcontent.html
Create a Web Clip Bookmark Icon
iPhone and iPod touch allow a user to save a Web Clip bookmark to your site on their Home Screen.
To specify a bookmark icon for all pages of a web site, place a PNG image named “apple-touch-icon.png” at the root directory of your web server – similar to the “favicon.ico” for site icons.
To override the site bookmark icon on a specific web page, insert a <link> element similar to <link rel=”apple-touch-icon” href=”/customIcon.png”/> within the <head> element of the page.
The bookmark icon dimensions should be 57×57 pixels. If the icon is a different size it will be scaled and cropped to fit.
Safari will automatically composite the icon with the standard “glassy” overlay so it looks like a built-in iPhone or iPod application.
Apple recommends the size of the icon as 57×57 pixels, but as we all can see, Apple’s own Web Clip Bookmark Icon is 129×129 pixels.