I am running Gentoo Linux on my laptop and yesterday I installed all of the updates available for it, which were roughly 2 weeks worth. Among these were a xorg-server update to 1.8.0. Following this update, my KDE battery monitor plasma widget has stopped detecting my laptop's battery.

Could this be related to udev replacing hal in xorg-server 1.8.0? Does anyone have any idea how to fix this?

I just re-added hald to my default runlevel and rebooted. Suddenly the battery monitor magically works.

I will try to file a bug report when I find time.

KDE's Solid framework currently only has a HAL backend, thus the battery applet was unable to find any batteries or chargers since HAL wasn't available.

Changes required in the Solid architecture to prepare it for compatibility with the new udev + upower + udisks + etc framework will be present in KDE 4.5, although actual backends for them won't be unfortunately, as they haven't been written yet.