H & H Color Lab (a professional print lab) has a blank PS design guide template for the iPhone 4/4S you can download here -
http://www.hhcolorlab.com/create/spe.../iphone_4_case. I've used it to create iPhone cases for clients before even though I use a different lab. I've also gotten quite a few iPhone templates on Etsy...Hazy Skies Design, Birdesign, Oh Snap are a few I can think of right now.