I've been fiddling with this problem for a while now.
I tried installing Combo updates, removing BezelServices.plist preferences, Power Unit Reset or something like that, playing with light and dark areas, covering the speaker holes (thus the sensor).. It all comes down to the same problem..
After having closed the lid of my 15" Powerbook (going into sleep mode) and reopening it again after about a minute or two my keyboard backlight no longer functions. Only if i restart my laptop completely does my backlight function normally again.. various people seem to have the same problem but haven't found a solution yet.
Please help, i'd like to wake my powerbook and have a working backlight in my keyboard..