One of the objectives that the solid team is currently working on, is the deprecation/removing of HAL as KDE dependency. We hope to accomplish it for KDE 4.6, but of course at the moment it is just that, a wish.
Anyhow the complete remove is now a step closer, thanks to Dario Freddi and Lukas Tinkl I just committed the "PowerDevil 2 UPower" backend. Right now, it is build by default but is not used if the hal backend is compiled too, so in case you want to test it you have to edit powerdevilupowerbackend.desktop and set InitialPreference = 20 (lower than the hal one).
Small entry for a small thing, but small things matter don’t they?