Having owned macs since 2008 i would say the apple tax most certainly DOES exist, but it pays for the development and upkeep of what shortly be (once windows 8 kills Windows as a desktop os) the only mainstream usable desktop OS.
Macs are NOT made with higher quality components, they are made with the cheapest stuff apple can get away with, apple are just better at hiding massive component failures from the buying public (1st gen time capsule, shoddy capacitors, thousands of failures, Nvidia GPU farce, thousands of failures, Ongoing issues with iMac screens not being sealed properly and sucking dust in between the LCD matrix and back light, iPad/iPhone/Macbook screen yellowing issues, the list could go on and on)
The Macs Cases, are quality machined components, but the insides, they are made from the same stock as the equivalent, and usually much cheaper PC.
I own apple, I choose to own apple, and pay the apple tax, because to me, the apple tax is the same as the Microsoft tax i used to have to pay, its the cost of an operating system i want to use.
but i will say one thing, one very very important thing, if your buying an iMac, buy APPLE CARE, and make sure you back everything up via either an external drive or a dedicated time capsule, because, like all PCs, its not a case of if it will fail, but when, and the all in one, none user repairable nature makes that when = buying a new iMac rather than replacing a failed component.