Well, cpuspeed in included (in FC3 it's part of the kernel-utils package, for FC4 this package will apparently be split into its components) and it should do what you want. Since kernel-utils is a pretty important package I'm quite sure that you'll already have it installed. You might want to tune the parameters in the /etc/cpuspeed.conf file to get it to respond the way you want; running '/usr/sbin/cpuspeed --help' will give you some instructions for what to put in that file. Use chkconfig to see if it's being started automatically and to turn it on if it isn't.
Interestingly, when I was using my laptop in bed this morning (blocking all ventilation ports with duvet), FC informed me that the processor was over temperature, and that it was switching to "modulated clock speed mode" or somesuch.