The iPhone 4 / iPad 3 and above need a 114px x 114px icon.  You can create separate resolution icons for each device type, but creating a single 114x114px icon is the easy option and allows it to be scaled for other devices.

Save you icon in the root directory of the web site named as:

apple-touch-icon-precomposed.png – if you have added your own apple effects

apple-touch-icon.png – if you want apple to add the shine etc effects

N.B. The file must be present in the sites root directory even if you app is in a sub directory.  If you can't do this there are ways to specify the file location within the html.

Further Information