Most of the plasma manufacturers now claim that their burn-in resistence is about equivalent to that of a standard CRT television. By the way, CRT's will burn-in, but typically over the course of 10+ years.
I bought a 42" LCD about a year ago specifically because I knew the kids would be playing video games on it and would, at various points, pause their games and leave them on all afternoon. However, I wouldn't hesitate now to purchase a plasma and most people like the plasma picture better for viewing.
From what I have read, Sony, Panasonic and Pioneer all score well for burn-in resistance.