Here is a CNET article on burn-in I came across while looking at this same question.
Quite curiously, it is dated exactly around the time this question was asked here :-)
CNET Article question: Is plasma HDTV burn-in a problem?, Geoffrey Morrison , August 15, 2011
Here's the most important fact about image persistence: unless you're overtly negligent, it's easily reversible. With what magic, you ask? Easy, just watch TV. Yep, that's it. Just go back to your regularly scheduled programming (full-screen, non-letterboxed television) and it will go away by itself. Just due to being used, the phosphors will get back in line. Depending on the severity of the image persistence, it may take a few minutes or a few hours to go away.
Modern plasmas have better phosphors that are less likely to "burn" in the first place. They also include features designed to lessen the chance for image persistence or remove it if it occurs. An orbiter function moves the image around the screen by a few pixels. Hardly noticeable, but it minimizes some aspects of burn-in. Full white or rapidly changing colored patterns excite the phosphors evenly, greatly reducing the time it takes to remove the effects of image persistence
All that being said, I looked up a bit on pixel orbiter and found that people are not too impressed with that either. I'd still like to know if there are any good references on if and exactly-how this helps, and do most recent plasma TV models have this feature.