Burn in is a bit of a misnomer. Differential phosphor aging is more correct. The parts of the screen that get used (the center when displaying 4x3 content) age, and the parts have no image don't age.
The gray bar thing was designed to prevent this by "using" the phosphors that were otherwise...
Thank you, Adam, for quoting only the part that suited you.
My point regarding TV is- there are many series (ER, West Wing, etc) that are shot widescreen, and that aspect never sees the light of day. At home, with my stretch modes, I have a choice, made by me, for me. No one else affected...