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I wouldn't count on the resale value of Mac hardware holding. I think one of the reasons you could count on it over the years was the amazingly closed market for Mac hardware.
Off the top of my head:
1> Product refreshes were slow and unsteady. Often the new model wasn't that much better than the one it was replacing. For someone on a product 2-3 years old your 1 year old system is a good upgrade.
2> Extremely constrained supply. When Apple stopped making - that was it for that product. No way you could cobble your own Mac together out of OEM parts.
That's changing rapidly. Apple's move of the Mac to essentially commodity hardware erodes some of the value holding capabilities of the platform.
For instance:
1> On the standard x86 PC (Windows/Linux) there has been immense market pressure by competitors. AMD/Intel, ATi/nVidia, and a plethora of OEM manufacturers of motherboards, etc.
2> 6 - 12 month product refreshes mean that literally in a year something that's truly better is really available.
By moving to x86 hardware - and ignoring the irrelevant slam on Windows getting "gunky" after a year (dunno about you but my friends and family don't seem to have that issue)... I've found the problem is related to naive users - 2 heavy Mac users I know found themselves reinstalling OS X about yearly to decruft their systems as well...
In any case by moving to commodity hardware - people will expect it to get better every 6 months. Apple is entering the volatile market (to a degree, sort of like how China participates in capitalism by maintaining a "walled garden").