Finally cracked this.. I believe it was caused (mine anyway) by SPB Benchmark. After looking back, I realized the times matched. Only have a few mins right now, but I'm pretty sure it's caused by the battery tests.
Check your registry,
HKCU\ControlPanel\Backlight
Look for keys
OnOff or
OnOffAC.
If either of these are set to 0, the brightness sliders are completely byp*****. I believe (in my case) SPB Benchmark set the OnOffAC key to 0 in prep for the battery test I was going to run... irony is, after seeing how long it would take, I cancelled the test. But the app didn't restore the key. Since this only happened to me on external power, it took me a while to notice... and a day an' a half to lock it down.
Questions / comments? I'll have more time tonight.
G'Luck!
