After having the screen on my iPhone 3GS replaced (even though an AppleCare rep told the Genius to replace the iPhone), I ended up dissatisfied with the screen replacement. There are scratches and knicks across the screen, and after further examination, there are a few pieces of dust under the screen as well. Should a problem with the screen/glass justify an iPhone replacement? It can be difficult to replace a screen in a regular store without leaving some kind of marks and dust under the screen. Even the smallest mark under the display can get annoying during normal usage. I'm sure that it's rare for a Genius to do a perfect display replacement, so I think that an iPhone that has display/glass problems should be replaced if it's under warranty.