You can transfer your existing AppleCare to a new computer I think. If not then you should be able to get a refund for the remainder time of the warranty and apply that toward a new AppleCare warranty.
If this is a replacement computer because of a warranty issue, I would guess you could receive either a pro-rated refund or Applecare would transfer over. Apple would take care of this.
If this is an additional computer, you would have to buy a second Applecare policy...just like you'd need to buy a second insurance policy if you bought a second car.