If you have a clear case you can put a picture behind it .. getting the dimensions just right and not cutting anything off can be tricky ... So I was looking for a template shaped and sized like the back of the phone with cut out and all.
this is kinda tacky but why not become a fan...download what you are looking for then either let the updates go wayward with the way facebook is working or just unfan them? It's shady I know but it happens...happens on my page daily lol (assuming the fan fave is on facebook)
I agree with Meg, just become a fan and call it good ... or you can easily convert a flattened image by using your magic wand tool (contiguous unselected) and clicking anywhere on the background and hitting delete.
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/specialty/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.