True, but I would think you could get a 3-4 year expectancy out of the current iMac's. Hell my 3 1/2 year old eMac still does a good job for basic computing.
Maybe my iMac will have a 10 year life!! Though I intended to keep it for 4-6 years, I'm looking at a macbook pro for college. Also, the more powerful the computer is, i bet its been mentioned before, Apple will roll at a new operating system, and eventually, Apple will require you, for example to have at least a 2.1ghz core 2 duo processor (for example), with a faster mac, you can have more to date specs that will serve future OS releases and high end programs, also one of the reasons i got this machine, but I'm sure the 2.4ghz, or even 2ghz can handle it.