I've also noticed that my Macbook Pro boot sequence only occasionally ups the brightness to full when rebooting. (However, I reboot so rarely that I don't really mind.) I thought it was just me! I'm running OSX Lion (10.7.4), btw.
I like the suggested answer that the user preference value isn't read soon enough, but I suspect that the issue is that the appropriate driver isn't loaded until the last moment. From what I understand, the user prefs would be readable at any time during the boot sequence (assuming the root/boot disk volume is mounted, anyway), but if the driver isn't loaded, the value wouldn't be applied. That would account for the display being really bright for some time, but it doesn't explain the occasional (for me, about every second bootup) dimmer value.
Of course, if the dimmer code is part of the BIOS, that's completely wrong, and it probably would be down to when the user prefs were read and fed to the driver!
The strange unpredictable behaviour may depend on what sequence of events occurs when shutting down; it's possible (though not probable) that in some situations, the brightness register isn't reset when shutting down. This probably wouldn't happen if the machine was off for some hours/days/weeks. It would be interesting to see if there's a pattern to the behaviour though!
It might be possible to re-order the sequence so that the display brightness setting is applied sooner rather than later, but it will still be some number of clock cycles between the LED backlight being turned on and the appropriate driver load/preference read.
The issue is caused by a burnt out soldering attaching a cable to an iron bar component situated in the lowest part of the screen. It is part of the module named 612-0035-a. It is the LCD inverter. It is not user solderable with standard equipment.
There's no need to buy an entire screen to fix this, just a new LCD inverter. To prevent same damage from happening again, it is recommended to brace from power surges.
Best Answer
Since you have already un-checked the "Auto-adjust Brightness" option in the display settings, try resetting the System Management Controller (SMC). This may resolve the issues related to power management. Follow these guidelines to reset the SMC. One of the indicators that your SMC might need to be reset: