Let's take a look at what happens when other Free Software operating systems run on my laptop, in terms of power consumption (and nothing else -- I'm not going to explain in detail what's running on each one, and rest assured that the available apps and toolsets in each of these installs is very different, reflecting what I use each OS for):
- OpenSolaris, display off, nvidia drivers: 31W
- OpenSolaris, display on, idle, nvidia drivers: 38W
- OpenSolaris, display on, disk + network activity, nvidia drivers: 52W
- Just the power brick: 1W
So it looks like Solaris is marginally (2W) better with power than Kubuntu at idle, and this laptop draws quite a lot in regular use. Maybe Solaris is not switching some hardware feature on, like Bluetooth, rather than being more efficient -- but I haven't noticed anything missing (Bluetooth is not something I'd miss). FreeBSD 8 fares no different -- roughly same usage numbers as the other two OSsen.
Many commenters suggested trying the nouveau driver -- so I did, whatever is available on Kubuntu 9.04 with no updates applied, and it makes no difference in power consumption, fails utterly at resume-from-suspend (nv got me no video, but I could ssh in, while now it just hangs), and is just as slow on logout as the nv driver. Perhaps there's newer versions available -- not something I'm going to experiment with this week.