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.