It is, indeed, very similar to a car cooling system, except that it has a much smaller radiator, fewer, smaller gauge hoses and pipes, the coolant (probably) doesn't get as hot, the pump moves far less fluid over far shorter distances, and it doesn't have to roll down a less than smooth surface at 80+ mph, sustaining impacts from rocks, bugs, and god knows what else. And yet, I wonder how many cars suffer coolant system failures within the first 3 years. A pretty small percentage, I'd wager. Sure, it is absolutely possible for the liquid cooling to fail on the G5, but it is almost certainly not likely to happen within the time where the computer is useful. If you are unlucky enough to be one of the folks under the tail of the bell curve, you could get screwed, but the odds of it being you are pretty small. You are probably much more likely to suffer any number of more mundane failures which contribute far more to the cost of applecare than potential cooling failures. If you are worried about cooling failures, you should be far more worried about hard drive failures, power supply failures, motherboard shorts, power surges, etc. And if you are worried about those things, then you should already have paid for applecare. A failed cooling system is a non-issue, statistically, as far as I can tell from the evidence that is public so far and by looking at the example of automotive cooling systems.

--sam