I'm a big fan of Applecare for computers (though not for iPods) - to my way of thinking, it gives you peace of mind for three years.
I have had Applecare on each of the two Apple computers I have owned (a MBP and a MBA) and have actually had to avail of it for each of them. Well worth it. It meant that when the HDD went on my MBP, two years after I bought it, Apple replaced it without a problem.
Likewise, 14 months after I bought my MBA, the magsafe adaptor needed replacing, and the keyboard - which failed - was also replaced under Applecare.
There are those who replace computers every year. I'm not one of them. However, having Applecare when you sell on means that potential buyers may well be reassured even if they are buying a computer which is over two years of age.
Re Applecare, you can buy it at any time during the first year you have the computer - one year from the date of the sales receipt - but not one minute after that. Personally, I recommend it, especially if you plan to hold onto a computer for longer than a year.