Binary package hint: hal
I've looked at all the various thinkpad-brightness-related bugs on launchpad, but this seems to be one that hasn't been reported yet. On my Thinkpad T41 (model 2668-F5G), when I press the 'brightness up' (Fn+Home) key, the screen brightness goes up by one step as it should, but gnome-power manager doesn't recognize the keypress - the popup display doesn't appear, and the slider in the brightness applet doesn't go up. On the other hand, when I press the 'brightness down' (Fn+End) key, the screen brightness goes all the way down to the minimum brightness - but the popup display does appear and the brightness applet's slider does go down to 0. This was all with GNOME; in KDE, there isn't the brightness-dropping-to-zero problem, but the 'brightness up' keypress still doesn't get recognized.
I've filed this under hal, but there's probably a gnome-power-manager aspect too.
Never mind about this; I've traced the root cause of this problem in hotkey-setup and filed it separately as bug #221571. Because of that bug, the hotkey mask doesn't get set correctly, so the 'brightness up' keypress isn't received by hal which causes the problem here.