The only 'maintenance' issue is one called Screen Burn - plasma displays dont like static images or part images (TV channel logos) being displayed for long periods of time; you can end up 'etching' the logo or fixed image into the display permanently.
Normal viewing of a mixture of video sources and proper use of the various 'screen saver' modes provided by many plasma manufacturers will ensure you have a 'burn' free screen.
If anyone mentions re gassing a display leave the shop - they dont know diddly squat about plasma displays.
Expected 'useful' life of the latest fifth generation displays from the likes of Pioneer and Panasonic is in excess of 25,000 hours of viewing time.
Right, im no expert on plasma's, i havnt even bought mine yet (getting it in new year), but im going to have a go at answering your question
Plasma's dont just stop working, they just deteriorate. The colour starts to fade over the years. In say 5 years, the contrast will be half of what it was when you bought it. You probably wont even notice it as it would be a gradual thing.
Another thing you might notice in a few years time, is a couple of pixels will fail, they will be permanently lit in a colour, but they are so small, you probably wont notice them either.
If anyone things i have given the wrong answers, please dont hesitate in slating me