Though it is still very much in the realm of possibility that firmware/software could be the root of the problem, all I know is that on my SP1, the dimming was bugging me to death and I thought I had a defective unit. Moreover, I didn't remember it always happening; consequently, I then started to believe that it was a result of a firmware update.
By the grace of God, I eventually stumbled on a thread on this site that discussed the Auto Dimming feature and that's when I discovered that setting and was able to disable it. Once disabled, I never had that dimming problem again.
For someone who likes maximum brightness all the time (i.e., plugged and unplugged), you don't know how grateful I was to find out about that setting!