The HP thin clients use VIA processors, for low power goodness. I can use the longhaul cpufreq driver without a hitch on my t5000 as it uses the VIA C3 chip, but I can't get anything to work on my t5135, as is uses the newer VIA Ester C7 Eden chip.
I've done a bunch of research and some have said it might be available in 2.6.21, but its not there yet. The most promising candidates are longhaul and e_powersaver. I'm just looking to keep the temperature down when possible.