I've managed to activate CPU frequency scaling of my SI1520's Core 2 Duo processor in Ubuntu Feisty.
Everything passes through the CPU Frequency Scaling Monitor Gnome applet, which must be reconfigured to enable manual CPU frequency scaling.
The shell command to do this is:
$ sudo dpkg-reconfigure gnome-applets
You will have to answer “Yes” to the question regarding setting the suid of the cpufreq-selector executable. Now you can left click on the CPU Frequency Monitor Applet and choose the CPU frequency and/or operative mode (Conservative, Ondemand, Powersave, Performance).