Try installing udev-git from the AUR. I had a similar problem with the Fn key in combination with the F1-F12 keys on my laptop; X wasn't registering any keypress events for those keystrokes. A thread on the forums about a related issue [1] suggested trying udev-git and it solved the problem for me.
udev-179 (which is the current version in Git) is in [testing], so the official package should have the fix fairly soon. [1] https://bbs.archlinux.org/viewtopic.php?pid=1047167
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