Like Eric said, the warranty is mainly for the hardware support. Sure, you get phone support for the OS, but I honestly don't think it's worth paying for AppleCare just to get phone support when you get much better advice for free at places like Mac-Forums.
If you have a desktop Mac, I find AppleCare to be a waste of money as Macs are pretty well built machines and the one year warranty should cover any manufacturing defects. A notebook is a totally different matter. Since they get moved around and see a lot more abuse, AppleCare is a great idea as there is a possibility of hardware breaking down for causes beyond manufacturing defects. Three years is a pretty good coverage time for a computer. If you buy it with an American Express card, you automatically get the warranty extended to four years.