For what it's worth, it is in commit
495f78bd6d8f7a5e35dd962031eb6e639d83e438, which IMHO should be reverted:

   UBUNTU: Build in CPU Frequency scaling drivers
    
    Selecting the right CPU Frequency scaling driver is complicated from
    userspace, involing a nasty shell script that attempts to guess by
    grepping through /proc.
    
    The kernel drivers themselves can adequately determine whether they
    should be used, building them into the kernel will automatically select
    the right one.
    
    These aren't something you would want to unload either, you would
    instead simply change the governor.
    
    rtg - Added debian/abi/2.6.28-8.23/modules.ignore to accomodate the missing 
modules.
    
    Signed-off-by: Scott James Remnant <sc...@canonical.com>
    Signed-off-by: Tim Gardner <tim.gard...@canonical.com>


- First of all, the kernel does not (and simply cannot) always adequately 
determine which one is the best driver to use for someones hardware (especially 
when choosing between speedstep-centrino and cpufreq-acpi). And even if it 
does, so would lets-modprobe-just-about-any-driver do. 

- They are something you would want to unload in realistic situations, 
especially since:
  * Some people prefer NOT to have a cpufreq driver, that's why it is a config 
option in the first place. There are systems known to consistently trigger a 
"pending-bit stuck" when using the powernow-k8 driver.

  * AFAIK the Ubuntu team wants to achieve a faster boottime, not
slower. Compiling in all drivers slow down booting noticeably. You don't
need them to boot either, so they can be postponed until a bit later.

  * Newer processors may not be supported until a new kernel is
distributed, unless a custom module can be loaded. This is true for AMD
0xf family processors with more than 1 low power state. Also for all
Black Edition AMD processors, the powernow-k8 driver does not support
using their unlocked multiplier and probably never will because AMD
doesn't want that feature in their powernow-k8 driver. The Linux-PHC
project provides such drivers that can be installed/maintained by the
DKMS.

  * You may want to use a modified module, that enables undervolting, to
maximize their battery life or reduce extreme heat by reducing the
processor's power consumption. The Linux-PHC project provides such
drivers. There is a large audience that uses them...

-- 
acpi-cpufreq/powernow-k8 should not be built-in into the kernel image
https://bugs.launchpad.net/bugs/355232
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