The trick to get iMacs cooled is that the monitor is directly over the motherboard. All the heat from the monitor draws a lot of air up past the board. Also, G3s don't generate much heat.
The Cube could do without a fan because of a couple of things:
a)External power supply. Possible because you couldn't put anything in there to suck up extra power. If three hard drives try to spin up all at once, or some such thing, you need the power supply much closer to the action.
b)Low clock speed. What do those run at, about 400 MHz? Not all that fast, anyway. With G4s running at 8-900 MHz now, Apple has had to make the heatsinks on those things absolutely huge, just to keep them cool without their own fan right on top of the chip.
Who knows, maybe the G5 will need its own CPU fan. Then the days of forgetting wether or not you turned off the computer will be well and truly over...
Personally, I'd really go for a computer, (lower power & lack of expansion would be OK) that could run without a fan, but also do two monitors. Like an iMac that could do more than just video mirroring.