This has gone far enough, I guess I've been a little obtuse, I've suggested trying other distro's so that you can come back and report if others have solved the problem.
Ubuntu is a community that depends on it's members to help make things better, if you can try other distros, and compare the differences between them and Ubuntu, and create meaningful bug reports, maybe this problem can be resolved before the next LTS release.
My fallback distro is Precise, and perhaps 13.04 Alpha will come out and I can shake 12.10 off my foot. I'm still reporting/following 12.10 bugs in the meantime. I'm on holiday using my daughter's MacBook (hardly know how) and will get back to looking at B2 when we're home.
I think that since this is happening on bog-standard hardware as well, a warning should be added to the release notes.
Good news FYI: When switching from the nouveau drivers to the proprietary nvidia-current, power consumption is reduced by ca. 5 W in idle from ca. 20.5 to ca. 15.5, and around 18 W when stuff is running and using 30% CPU plus frequent disk accesses.
With that consumption when using nvidia-current, Quantal is back in the ballpark of previous Ubuntu releases. So that's good. (And in addition, nvidia-current makes suspend work, which is another thing currently broken with nouveau (LP #1055475)).
However, in Quantal, nouveau seems to be the default for Nvidia cards. And nvidia-current and nvidia-experimental-304 are currently not practical on my MacBookPro because backlight adjustment is broken (another regression). This affects also more usual hardware, see e.g. LP #1046088
backlightx will work for you
run ls /sys/class/backlight/ to see if the where line is right
my backlight works for me using nvidia's drivers but this is not a mac