The "good" resale value comes into play the longer you hold onto it. There's always a steep dropoff in value at first. But for example, I paid $900 for a macbook in 2006. I sold it last week for $450, and could have gotten more, but priced under market so I could sell it faster. That's pretty good for a 4+ year old system that has been heavily used.
Cheaper systems do hold more of their value more than the higher end systems, tho.
This is absolutely correct. I don't "waste" my money on keeping up with updates. I paid $1600 for my 2.5GHZ Penryn 15" MBP w/Matte screen from apple.com the day the unibodies were released. Apple gave me a new one due to many problems with mine.
So lets say mine was worth $800 with AppleCare (my old one). That means I would have had it for 28 months and got $800 use out of it. Whats that equal, $28 a month? Not too bad if I say so my self!