Curio: You can get 4S replacement screens/digitizers for
cheap. Like the 5, they're pretty simple to replace provided you're technically minded. There's about 27 little screws that need to be taken out to replace the front assembly, which is fine but because they're all different sizes (and you need to put them back in the right place!) you need to make sure you've got some sort of filing system to store them in while you're working on it.
Mimi: The front glass will be glued to the touch screen/digitizer. You *can* separate them, but it's far more hassle than it's worth and is likely to cause future problems with the phone as the replacement will almost certainly not stick on properly. You should always replace the whole shebang. Unfortunately, unlike the 4S with cheap parts available, the 5 still looks expensive - I'd take it back to Apple and see what they say.