Web Clip Bookmark Icon Template
It seems that while Apple says to build icons at 57×57, the iPhone cuts off 1 pixel on the left and right and 3 pixels at the bottom the icons. It also rounds the corners and overlays a “glossy” appearance . Many people have posted that they created icons with larger dimensions to solve the problem, which makes for a larger download for users.
The good folks over at iPhoneminds have created a nice photoshop template for web clip icons, which includes the overlay, rounded corners and guides to help you find the exact center of your icon.