@Husse: this link concerns only CPU that work with acpi_cpufreq such as Core Duo/Duo2 but not earlier generations of Pentium M like Sonoma or Dothan. Mine is a Sonoma (single core) and it uses speedstep-centrino.
@Lord Plain: I tried to add "blacklist speedstep-centrino" to /etc/modprobe.d/blacklist and it effectively prevents the speedstep-centrino module to load after a reboot. However acpi-cpufreq module is not loaded automatically and even if I type sudo modprobe acpi-cpufreq to load it manually I get:
FATAL: Error inserting acpi_cpufreq (/lib/modules/2.6.24-16-generic/kernel/arch/x86/kernel/cpu/cpufreq/acpi-cpufreq.ko): Device or resource busy
Any solution to that? Indeed it would be perfect if I can load acpi-cpufreq
EDIT: with speedstep-centrino blacklisted, I can load acpi-cpufreq but first I have to manually unload speedstep-centrino with sudo modprobe -r speedstep-centrino. Then I can load sudo modprobe acpi-cpufreq and it works. It seems that blacklisting speedstep-centrino makes it uneffective but it's still loaded.
Is there a way to force speedstep-centrino to not load at startup and load acpi-cpufreq other than with a script??? That would do the trick.
EDIT2: seems like adding acpi-cpufreq to /etc/modules loads it at startup and it works with speedstep-centrino blacklisted.