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I just setup CPU frequency scaling in Gentoo on my Thinkpad T61p (Intel Core 2 Duo). These are the steps I followed:

1. Enable relevant kernel modules/options:
   #
# CPU Frequency scaling
#
CONFIG_CPU_FREQ=y
CONFIG_CPU_FREQ_TABLE=m
CONFIG_CPU_FREQ_DEBUG=y
CONFIG_CPU_FREQ_STAT=m
# CONFIG_CPU_FREQ_STAT_DETAILS is not set
CONFIG_CPU_FREQ_DEFAULT_GOV_PERFORMANCE=y
# CONFIG_CPU_FREQ_DEFAULT_GOV_USERSPACE is not set
# CONFIG_CPU_FREQ_DEFAULT_GOV_ONDEMAND is not set
# CONFIG_CPU_FREQ_DEFAULT_GOV_CONSERVATIVE is not set
CONFIG_CPU_FREQ_GOV_PERFORMANCE=y
CONFIG_CPU_FREQ_GOV_POWERSAVE=m
CONFIG_CPU_FREQ_GOV_USERSPACE=m
CONFIG_CPU_FREQ_GOV_ONDEMAND=m
CONFIG_CPU_FREQ_GOV_CONSERVATIVE=m

   #
   # CPUFreq processor drivers
   #
   CONFIG_X86_ACPI_CPUFREQ=m
   # CONFIG_X86_POWERNOW_K8 is not set
   # CONFIG_X86_SPEEDSTEP_CENTRINO is not set
   # CONFIG_X86_P4_CLOCKMOD is not set

   

2. Add the appropriate modules to /etc/modules.autoload.d/kernel-2.6 so that they are loaded at boot time:
   # cpu frequency scaling
acpi-cpufreq
cpufreq_ondemand
3. Install cpufrequtils:
   emerge cpufrequtils
4. Set CPU frequency governor in /etc/conf.d/cpufrequtils:
   # /etc/conf.d/cpufrequtils: config file for /etc/init.d/cpufrequtils
# Which governor to use. Must be one of the governors listed in:
# cat /sys/devices/system/cpu/cpu?/cpufreq/scaling_available_governors
#
GOVERNOR="ondemand"
5. Add cpufrequtils to boot runlevel:
   rc-update add cpufrequtils boot
6. Reboot (or start the stuff by hand) and enjoy a more efficient system.