An earlier article described how you can customize the new Character Picker introduced with OS X 10.7 Lion, so that the popup menu that appears when you hold down a key can contain whatever characters you want.
This feature strikes me as more revolutionary that it might first appear. A custom popup menu can also contain combining diacritics (Unicode block 0300 - 036F). That means you can set up a single key which, when held down, will let you add any arbitrary diacritic(s) to any base letter you have just typed. This has the potential to be a lot easier than the usual US Extended and other layouts with their complex systems of option and option + shift shortcuts. Perhaps the use of those modifier keys to create special characters can be done away with altogether.