I think older generations claimed 30k hours before dimming. The new generations are "supposedly" lasting 50k hours. If that's true, it'd take me like 14 years to kill a plasma. And by then it'd be time for a new one anyway. FWIW
Projection (LCD and DLP): the bulb burns out after 8k hours or so, and the replacement costs roughly $300. Usually you can replace it yourself though.
I don't know much, but that's what I do know.