Apropos of nothing, do you have BOINC running on any of your machines? I installed it a day or two ago on my HP and on my work laptop. Already I've made a material contribution to some important projects that can't afford supercomputer time; something to do with a disease I think. Anyway, I'm a member of the World Community Grid and Einstein@Home, but I'm thinking about dropping the space stuff in favor of the terrestrial problems [ ... ].
The real question is, what's your take on this stuff. Am I contributing more to the energy companies than to the advancement of human knowledge?
I used to run SETI@Home on a few systems. I noticed that my laptop was always hot when I ran it, so I stopped using it there and then gradually stopped using it altogether. That may be BOINC-based now. I also tried to donate cycles to some Brit climate project a while back, but they didn't have Linux support.
Even on a non-laptop it does cost you power. Most current CPUs have frequency scaling, and use much less power when running at lower frequencies. Gnome has applets that let you monitor CPU frequency and temperature if your hardware supports it.
I don't know how to weigh energy use vs. benefit to mankind. It will cost more when run in an air-conditioned room since you pay the electric company to heat the machine and then pay the electric company to cool the room containing the machine. OTOH it could save you a tad on heating in the winter (at OSU we had an Intel hypercube that could easily heat a couple rooms). If your machines suspend after a couple hours of no keyboard or mouse activity, then the impact may not be very large. I'd be leery of running it on a laptop, especially if it keeps the machine from suspending (as SETI@Home did a decade ago).
Soundtrack: "Danger" by the Motels