Burn in, especially on Panasonics, seems extremely unlikely.
I have watched basically one channel for 95% of the time my television is turned on: Cartoon Network. A black and white logo sits on the bottom right on all shows, except for late at night, when it turns to the Adult Swim logo. After 4 or 5 months use, I noticed a slight grey outline of the logo in very dark scenes. Was it burn in? No. It was polarization. This was easily concluded after leaving the television on for 24 hours playing a different channel. The mark disappeared. Polarization is not permanent. I suspect that many people mistake polarization as burn in, especially on modern 1st tier plasmas, which appear to be (and are claimed by 1st tier manufacturers) highly immune to burn in.
I don't believe that 30 percent game time (specified by the poster) would be an issue on a high quality plasma display. If the display was used for 90 percent game play, and the same game was almost always used, then perhaps a plasma would not be the ideal choice.
-Chris