*oops...meant to post this in another thread*
My first 3g had dust under the glass. It looked like a dead pixel but in sunlight you could clearly see the dust. They replaced it with no hassle.
My second 3g had dust under the glass again. I had checked it when I got it and didn't see any but later on I noticed it in direct sunlight. I couldn't see it under normal operation but in sunlight I could. On my second visit back to the Apple store, we went through 5 replacement 3gs before we found one I could live with. They ALL had dust under the glass. The one I settle on had dust under the glass but it was tiny and off the edge of the screen.
At some point I have since somehow acquired another piece of dust under the glass. I don't really notice it 95% of the time but again in direct bright light it is as clear as day. I'm contemplating whether I want to bother replacing it again since I have no confidence that I'll ever get one that's dust free/dust proof and I don't want to have to once again replace my screen protector and the decal I have on the back of the phone, however I know myself and I have a feeling I will be getting it replaced again (glad I got the apple care plan).