The lifespan of LCD backlights and electron guns are in the 10K+ range so by the time you reach that, you'll probably skip the display.
LCD/DLP projectors (RP/Front) have bulbs which last 2000-3000 hours, at a cost of around £250.
In Plasma, and CRT TV's its the phosphor coatings that dim over the years not the electron guns, or plasma cells themselves.
A modern plasma will likely be rated up at 60K+ hours, which is just as good as a CRT, and will probably out live the backlight on an LCD panel as well. The reason CRT and Plasma get 'Burn' is because the panels dont get exposed to 'constant' brightness, so the 'dimming' effect can be inconsistant. However the longer the 'average' halflife for the screen the less noticable burn will be anyway.
Anyway, just because a plasma's reached its halflife doesnt mean its time for the bin, most people dont run at full brightness, or full contrast anyway, about half way on both is 'right' on my Panny plasma, calibrating using the THX calibration tool. So as my plasma ages and starts to dim, I'll be able to increase the contrast to compensate for some time.
Anyway.. 60k+ hours... thats almost 7 years of 24/7 use... At 6 hours a day usage that 60k halflife will be reached after 27 years, Im sure there will be an electronics failer LONG LONG before the panel is unwatchable.
Ok, if you played 1 game for a year, and never did anything else, you could probably burn 'scores/ammo level' graphics into the screen, but if you use the TV for movies, TV, and games, it will have a 'fair' average use, and should burn fairly evenly.
Current generation plasma panels have great colour, good resolution, superb contrast, long life..... Pretty much the best picture quality for a large panel TV. Sure if your have a 3-4 year old Pioneer, they did have serious burn problems (panny had already overcome most of the problems by then).. But Pioneer too have resolved the burn issue for the most part as well.