It takes a bit of perfection to make it look right but you can do this rather easily.
In most fps'ers you don't actually draw your charactor unless you're in some sort of cinimatic scene or something thats why you don't normally see the legs or body.
All you need to do is set it up like a third person camera where the camera's position is in the head of the model possibly modifying the model to remove the head so you don't get any bad effects from the camera being inside the model at any certain point.
In some games you can not see your charactor from the camera but you can in a mirror for example this is just texture effects or special code to show things where you actually want them drawn and not where you don't.
Keep in mind that ANYTHING is possible with 3d graphics aside from having too much poly or something however that doesn't mean its all that easy to do.
