I came back from the Apple Store with a spanking new iPhone 4 (warranty replacement).
First issue: the well-known proximity sensor problem affected me randomly even after the 4.1 update. I was somewhat impressed by the in-the-field debugging I saw at the Genius Bar. After recovering some debugging information from the iPhone itself, the dude also pulled out a known good iPhone and tested the proximity sensor of my iPhone against it. They also had a sophisticated iPhone app to file these kind of defect reports back to Apple. Fortunately my issue was reproducible (my phone did not perform as well as the known good one).
Second issue: scratched glass! My original iPhone still has a flawless screen and in fact I was excited when I saw in their promotional video that they bit the bullet and decided to engineer both front and back of the phone as glass.
When I visited the Apple Store I was aware only of a scratch in the front, but upon close inspection I noticed that a huge scratch had developed in the back too! I already knew that I was not the only one experiencing screen scratches.
I am very happy to say that Apple clearly stands behind their products.