Comment 29 for bug 543294

After some investigation, it seems like the actual bug has nothing to do with keymap.

Fn+F5/F6 actually does work. Keypresses are recognized as ACPI events (see comment #1 above), and keymap does not show anything ever.

Currently brightness level goes something like this:
Left hand side: what NotifyOSD tells me
Right hand side: actual brightness, compared to what I see in Windows (approx)

50% = very bright - probably 100% -- cannot go above this level!
40% = about 30% brightness
25% = about 50% brightness
10% = about 70% brightness
0% = about 10% brightness

And then, after reaching 0%, when I press brightness down again, Brightness control stops responding to any of my input (it seems to "freeze" for a while).

So the current workaround would be to make sure that you don't press Fn+F5 too many times and just live with mismatch of brightness levels.