It's all very well showing us pretty graphs but what I'd really like to know is why. There is nothing inherent about newer software that will make it use more power on older hardware, except maybe setting -mtune to a more recent processor. It may come down to just one or two changes.
I too am interested in the "whys". I disagree, though, with the assertion that there is nothing inherent in newer software that makes it use more power on older hardware.
By it's very nature, newer software uses more features of the hardware- graphics acceleration, multiple cores/threads, etc. Given the results, I have to wonder if the idle load on 12.04LTS is much greater than it was on 10.04LTS. If so, the Pentium-M is probably spending a lot more horsepower just to do nothing.
Ubuntu 12.04 is starting to shape up to be the same wort of watershed release that Mac OS X 10.5 was- one where whole generations of hardware are left behind because they can't keep up.
Let me guess: The "older Intel Pentium M notebook" has a Radeon card, probably in the X??? series... 10.04 came with KMS by default, and radeon KMS at the time had no power management, and was in this sense a regression from 8.04 and UMS. However you had the possibility to force UMS using radeon.modeset=0.
Improvements in KMS power management might account for the power consumption reduction from 10.10 to 11.04.