I own an iPhone 4 since mid 2010. I realized that, when it was new, the screen could easily be cleaned. Usually I was able to remove finger prints by simply wiping the glass with my heel of hand. However, in the meantime, it became worse. Now, it is hardly possible to clean the display at all. To remove finger prints I have to clean the glass by using a slightly moistened tissue (which I must exchange several times). When trying to clean it with a dry tissue, the finger prints become smeared over the glass like a film of oil. It is really annoying, because you can see the smudgy glass even if the display is on. I remember that the 3GS' anti-fingerprint coating was more persistent. It seems that iPhone 4's fingerprint coating sloughs within a few months of daily use.
Furthermore, my iPhone display gets more and more weird, little spots on the glass surface (it started on the bottom side of the screen close to the home button). It's not like scratches - it looks more like the surface becoming porous. Anyone seen that before? I'm thinking about bringing the iPhone to Apple's genius bar...