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CPU Frequency Scaling is a technology primarily for notebooks that enables the OS to scale the CPU speed to system and/or power use. For instance, slowing down the CPU when a notebook is on battery power, saves battery life. Intel calls this technology SpeedStep.The equivalent AMD technology is called PowerNow! or Cool'n'Quiet.

[edit] Steps

1. Install cpufrequtils

# pacman -S cpufrequtils

2. Load kernel module

Intel:

# modprobe acpi-cpufreq

AMD:

# modprobe powernow-k{6,7,8}

3. Load scaling governor(s)

# modprobe cpufreq_ondemand
# modprobe cpufreq_powersave

To load everything automatically at startup, add the appropriate drivers to the MODULES array within /etc/rc.conf. For example:

MODULES=( acpi-cpufreq cpufreq_ondemand cpufreq_powersave vboxdrv fuse fglrx iwl3945 ... )

4. Edit /etc/conf.d/cpufreq as root, selecting the desired governor:

#configuration for cpufreq control
# valid governors:
#  ondemand, performance, powersave,
#  conservative, userspace
governor="ondemand"

# valid suffixes: Hz, kHz (default), MHz, GHz, THz
min_freq="1GHz"
max_freq="2GHz"

Note: the min_freq and max_freq lines can be commented out, as the kernel driver should see your values automatically. To ensure it does run

# cpufreq-info

and check the output.

5. Start the cpufreq daemon:

# /etc/rc.d/cpufreq start

Add cpufreq to the DAEMONS list in /etc/rc.conf.

6. (Optional) Install and setup a GUI tool in your desktop environment. For KDE there is KLaptop and KPowersave. The latter is available via AUR and has more features. The devel-version is recommended, as it no longer depends on powersave.

More information can be found on the cpufrequtils page.
Notes for Dual/MultiCore processors:

1. Scaling will only work for the primary cpu0 core. Add these lines to your /etc/rc.local to make all cores scaling.

# echo "ondemand" > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor
# echo "ondemand" > /sys/devices/system/cpu/cpu1/cpufreq/scaling_governor
...more if needed

2. If the second CPU does not follow the frequency rules after suspend to ram, edit /usr/lib/hal/scripts/linux/hal-system-power-suspend-linux and add the respective line, e.g.

# echo "ondemand">/sys/devices/system/cpu/cpu1/cpufreq/scaling_governor

before the final line ("exit $RET").

[edit] Other resources

cpufrequtils - info for advanced users