Comment 1 for bug 162633

I've noticed that right after I installed gutsy and kinda know how it "works" now.

It doesn't reset to 100%, it resets to the value of "set display brightness to" slider in "on ac power" (even if currently running on battery!) tab of Power Management Preferences applet. It always happens after a few minutes from setting it with a "Fn+brightness" key. One thing that always trigges this is opening Power Management Preferences applet.

Workaround is simple: adjust your brightness using this slider and not the key combo. I started doing that some time ago and it's not that annoying.