Certain issues with screen brightness adjustment in Ubuntu for a variety Intel graphics chips (945GM, GL40) which will be fixed in the upcoming Ubuntu Karmic. For now, if you have problems getting your screen brightness adjusted via fn keys (I had this issue on a new Acer notebook), you might try this command:
xrandr --output LVDS --set BACKLIGHT_CONTROL combination
The screen will flicker when you run this but your function keys might just start working. You might add this command to your startup scripts. YMMV.