The PHC kernel for Ubuntu 11.04 Natty Narwhal is now available in the linux-phc PPA on Launchpad.
The Linux PHC project provides cpufreq kernel modules for undervolting Intel and AMD processors on a Linux platforms. This gives you the ability to save energy, lower the CPU temperature and expand battery time.
If you want to know more about Linux PHC take a look at the Linux PHC forum and the PHC wiki. The installation notes for the PHC kernel can be found in this thread. If you are going to measure the total power consumption, keep in mind the "10-30% increase in power consumption" bug.