I managed to crack the rear panel on my iPhone 4S. The one I cracked was already a transparent one from iFixit fitted, for purely cosmetic reasons, using an iPhone liberation kit.
Because I had already replaced the pentalobe screws with Phillips ones from the liberation kit the replacement was particularly easy. The whole process took well under 5 minutes. I was happy with the procedure having followed it before but the guide on this site is worth following especially for the reminder to remove the film over the camera lens cover. ;-)
The original screws have a small amount of threadlock on them which the replacements don't but I've not found any instances of them coming undone. If they do, I'll apply a tiny amount of low strength threadlock.