What process throttles CPU?

[Steve Snyder]

Yesterday, while doing a large build on an FC3 machine, I received 
many warnings from the kernel that the CPU clock speed was being 
adjusted due to the temperature beyond the threshhold.

Considering the machine I was using (a Dell Precision Xeon box), I can 
certainly believe the warnings.  I wonder, though, how the kernel 
knew the threshhold.

My first thought was the info beneath /proc/acpi/, 
yet /proc/acpi/thermal_zone/ is empty.

I then thought of cpuspeed, but the /etc/cpuspeed.conf file is just a 
sample and had only commented-out config parameters.

So... how does the kernel know what the thermal threshhold is for a 
given system?

Thanks.