My iPhone 4 is about 15 months old now and the screen has completely lost the oleophobic property is had when it was new: finger smears now show up just like they do on other phones. Contrary to the comments on this question I've never cleaned it with any sort of solution: all it's ever had is the occasional wipe with a cotton cloth (well, T-shirt :), although of course it will also have been rubbed in the pocket of my jeans. It's never had a screen protector but is completely unscratched and undamaged.
So it seems the coating does wear off over time without using any kind of chemical agent. My question is, is it possible (either at Apple, a third-party service centre or at home) to re-apply it and get the screen back to its fresh-from-the-box state?