I never had a screen protector on my iPod Touch 3G, but I'm considering it for my 4G. Does a screen protector affect the display in anyway? (make it dimmer or have any other affect on the display?)
Crystal ones will not (best options would be power support, or SGP). I think the majority of the other ones will at least a little. The only downside to crystal ones is that they are fingerprints and oil magnets.
I personally use power support anti-glares for my iphone. it takes away from the screen a little, but no fingerprints or oil spots. But if you are only interested in screen clarity, then Crystal protectors are the way to go.
I just wanted to update this thread. I ended up going with BodyGuardz. I didn't trust myself to install it correctly, so I went past a kiosk in the mall. I turned off the iPod and didn't touch it for about 18 hours. I didn't want to risk messing up the install.
I had the "full wrap" (front and back) install and I love it. It's been about a week now and you can hardly tell that it's there. The only way you can really "tell" is when you are swiping because you can feel the BodyGuardz material.
I have the KeyRing Rewards app installed and I haven't had a chance to see if the BodyGuardz has any effect.