You can transfer AppleCare to another machine of the same type (ie: You can transfer the AppleCare from a 15" MacBook Pro to another 15" MacBook Pro, but not to a iMac) as long as the second machine is still under the original one-year limited warranty; you cannot transfer AppleCare to a computer that already has AppleCare or expired warranty/AppleCare. Furthermore, you cannot buy AppleCare again for the second machine once it's been transferred over from another machine.
The gist of it: it's a crappy idea; you don't really gain anything if you only have say one year of AppleCare left since you don't gain anything on the second machine (since AppleCare replaced the original warranty).
Getting a prorated refund isn't that stellar either, if that's what you want, be warnedif you've ever used Apple technical support, repairs, etc., you will be docked the charges from the prorated balance. In the end you might have say 2 years of AppleCare left, but because you did 3 repairs and made 6 calls, you'll actually get no money back if you request a refund.
Now if you're trying to sell the machine, AppleCare would help with the sale; if you don't intend to sell the machine, consider it a good investment.
If your intention is to get a refund on AppleCare later, might as well not buy it in the first place; just sell your computer every year. Transfer AppleCare around only screws yourself. My 2¢.