I just witnessed what I believe is the first case of "floating blacks" on my VT20, and it was significant. It occurred during the closing credits of the movie "Training Day" from 2001 with Denzel Washington, Ethan Hawke and Scott Glenn.
The movie ends, goes to a black screen and the credits begin. It's not a rolling line of continuos credits, but one credit at a time (red text) that appears and disappears with a black screen in between each person's name.
Here's what happened...
Screen starts out dark black to begin with, then suddenly becomes a much lighter gray a split second before the first credit appears. It remains lighter gray while the credit is displayed, and for a second after it goes away, but then turns dark black again. It stays dark black for a second or two then becomes lighter gray right before the next credit appears, and for a second after it's gone, and then goes dark black again.
This pattern continued back and forth, for each credit, almost like a switch was being turned on and off with the black levels. It was very weird.
I've never encountered this before with any TV, but it reminded me of what I've heard some DLP and Projector owners describe as black "pumping" from a mechanical iris. Of course Plasma does not have an iris, so I'm assuming this must be the "floating blacks" I've heard people mention?