Perhaps there's some confusion about the different kinds of idle. An entire Untangle box isn't going to go idle like a laptop goes idle, and just shut itself off. But it probably has an idle CPU fairly often, as in
ACPI processor state C3. A CPU can go into C3 idle mode with a tiny fraction of a second of inactivity; even when I'm working on my laptop it spends a fair amount of time in C3.
PowerTOP is a good tool for seeing this, if your hardware supports enough ACPI for it to work.
@Evil Master, the Untangle kernel seems to be compiled with CONFIG_CPU_IDLE, so if the OS isn't doing anything it should tell the CPU to idle. Unfortunately my Untangle server doesn't seem to have very good hardware ACPI support and I can't find many details about what it's actually doing. PowerTOP gives good details, and there's lots in /proc/acpi/ that might be interesting.
There are probably lots of small optimizations that could save a great deal of power on Untangle servers, I'd be very interested to hear what you find.
Good luck!