The iPhone 3GS's oleophobic screen is most likely its biggest innovation in the sense of what will still be relevant about this device ten years from now. As crazy as this might sound at a first glance, it might dawn upon you once you have used a 3GS for a while.
The screen getting disgustingly greasy might be one of the most inherent flaws in the designs of the original iPhone and the 3G. Ask any user about this, they will agree that the way their finger grease builds up on the screen is not only repulsive to look at, but also makes it difficult to read the screen if the incident light comes at an unfortunate angle.
Not only that, but the original/3G screen also poses considerable friction against your finger as it slides over the touch panel. In fact, it is not uncommon for your finger to get "stuck" temporarily, especially during quick motions with dry fingers. You know what, even Bill Buxton cited friction as one of the prime problems with touch input devices in a paper published in 1985. Yes, 1985.
The 3GS's oleophobic coating alleviates both issues considerably: By covering the touch panel with a special layer that prevents fat and oils from bonding with it (this is what "oleophobic" means - literally "afraid of oil"). Also, you can clean the device by simply wiping the the screen once with a piece of cloth - since there's no bonding with the surface, the grease comes off immediately, no scrubbing and rubbing necessary.
Similarly, the natural fats on your fingertip form a little "cushion" upon which your finger can glide smoothly across the screen, rather than making it stick. Especially if you try both a 3G and 3GS side by side, you will be amazed by how much the difference is noticeable. In fact, you won't want to use an iPhone without this coating ever again, believe me.
Bonding the oleophobic layer to the screen was apparently not an easy task, which explains why the iPhone hasn't have this feature right off the bat. Nevertheless (or maybe even because, since this gave us better means to compare it) the oleophobic screen is the one iPhone 3GS innovation that might still be remembered in ten years from now: I suppose such coatings (and their future derivatives) will be ubiquitous on touch screens 10 years from now, and the 3GS will be remembered as the device that started it.