Yeah, and you get a pretty rainbow effect with the plastic "shield".
I'm not a fan of invisible shield. I haven't tried any of the alternatives though. But my experience with invisishiled just reinforced my thoughts that it was a lot like old people who put plastic over their furniture.
Wirelessly posted (iPhone 3G (white): Mozilla/5.0 (iPhone; U; CPU iPhone OS 2_0 like Mac OS X; en-us) AppleWebKit/525.18.1 (KHTML, like Gecko) Version/3.1.1 Mobile/5A347 Safari/525.20)
For my screen, I use a PowerSupport anti-glare film - not so much to protect the glass, but because it doesn't show fingerprints and it makes the screen easier to view on the train.
For the back, I tried InvisibleShield as I've been using it on every iPod I've owned since my 3G 20GB, and I like the peace of mind it gives. However, the I.S. for the iPhone 3G is a NIGHTMARE to apply. I went through two just trying to get it right (and don't forget, I've had LOTS of practice over the years) but I ended up ripping them both off in frustration. YMMV.