I went back and looked at glass screens for my palm pilots (I have 4 of them, one from 1997, a Handspring Visor from around 2001, a Palm Treo, a Palm Centro), the glass screens from my unprotected iPod 30g, ipod 80g, ipod 120g all of which were UNPROTECTED.
My Conclusion: glass screens for my 7 devices above GOT SCRATCHED UP LIKE HELL with use, and this happened to ALL of my unprotected units.
I've had Zaggs on my iPod 160g, ipod Nano 16g, ipod touch 16g, ipod touch 64g, my Kindle2 & KindleDX, as well as my most recent cell phones & my digitalSLR view screens - If you keep your phone in a highly protective case you'll probably be fine but if you're totally naked your screen WILL GET SCRATCHED, and you might regret it very much. Just trying to help.