With PowerDevil 2 and the resulting move to UPower from HAL, solid-powermanagement was removed. As such, this can no longer be specified, as UPower does not support it.

Unfortunately my CPU keeps running at full speed even when on battery (at least KDE's System Monitor tells me so). My battery drains pretty fast.

I'm running KDE 4.6.1 and removed HAL.

I checked the output of lsmod, and the modules "acpi_cpufreq" and "cpufreq_[ondemand | powersave | conservative | userspace]" are loaded.

Where else could I check my system configuration?

What does your module stanza in your rc.conf say? Have you got laptop mode tools and cpufrequtils installed? See the archwiki laptop page for more info.

For Thinkpads there is tp_smapi to give you access to almost all hardware features and a large percentage of key combinations.

Install laptop-mode-tools.

They allow you to configure power management completely independent from your desktop environment. It comes with very good default settings so in most cases it should be enough to install laptop-mode-tools and make sure acpid is running.

It controls cpu frequency, hard drive spindown, wireless power management and much more.