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I've been trying to figure this out for a while now, and it turns out it's an upstream bug with PowerDevil: https://bugs.kde.org/show_bug.cgi?id=205453
A patch was just posted on that bug report within the last couple days. I rebuilt kdebase-workspace with this patch and it's now working.
I've filed a bug report in the Arch bug tracker (http://bugs.archlinux.org/task/19426) making note of the solution. Anyone who doesn't want to wait for an updated package, and is familiar with ABS, can modify the PKGBUILD to include the patch from the KDE bug report.
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Poof, new package already. Nice!
This is why I love Arch.
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