I don't quite agree with the 4 month update, but it's pretty close in most cases.
http://guides.macrumors.com/Buyer%27s_Guide_%28time-ordered%29
I'd go there and look up how things are changing. I missed the boat on my first iMac (2006, still barely running with some help with my fixing), however I have known of some people playing the mac market just right so they can sell their current desktop or laptop sometime right before an upcoming update and get enough to only pay a fraction of the cost for a new one.
I also hear going and getting the extended warranty, wallet permitting of course, is good because it helps in resale value. Use computer, get an overhaul or fix a year or two down the line, sell it, buy new, etc.
All rumors and what not I can't confirm success rates.
Happy selling to you good sir.
Also I forgot the question all together so I am sorry about that. My main point is that with the dates of release and similar data you can gather from ebay as far as what models are going for what prices you can better assess what you have. So hopefully you can sell your macbook at a "peak" used value and not have to take too much of a loss.
Edited by ClemBurmingham - 5/8/11 at 4:42pm