I have the first-gen iPHone and now the 3G iPhone, and neither of them has any sort of permanent "shield" over them. The older iPhone I carried around in my pocket, usually in an In-Case case that left the front screen naked for easy access. Sometimes I would slip an Vayo leather back on it, again leaving the front bare. A year and a month later, the glass on that thing looks as good as the first day I owned it. Not a scratch nor a scuff...... With the new 3G iPhone I bought a Griffin "Wave" and promptly dispensed of the plastic covering meant go over the front of the iPhone. Next I decided I didn't like the section which acts as a sort of border. Now I simply snap the back section on to protect my pretty white iPhone's plastic back, slip the device into my pocket and away I go.....
Unless one is really careless and klutzy, prone to dropping or scratching up your phone, IMHO there is no real need to put the thing into an invisible shield or other sort of body armor casing.