The other thing that makes the Apple products hold their value is that generally they hold up better than their PC counterparts. I have had some Mac's and Apple II's that are pushing twenty years old and still fire up. I know apple had some logic board issues with the iBooks, but for the most part they are very reliable.
PC's hold a good value for the first year you own them. Shortly after that period the values drop of FAST. You can buy 3 or 4 year old Pentium 4 machines for less than you can get a 6 or 7 year old G3 B&W Powermac.
Also the demand for used Macs is pretty high too. They aren't as abundant as used PC's, which you can find anywhere.
My Two Cents.
The Amazing Resale Value of Your Mac