i've had my 50" plasma for a year now (Toshiba) and have had no issues whatsoever, however i've had mine set to stretch mode so when the kids watch, say, Disney or Cartoon in standard digital it won't have the vertical black bars as Damon mentioned (and it doesn't look too bad, at least Toshiba's version of stretch. Characters don't look squatty like a lot of stretch modes in other sets).
Having said this, a good pal of mine bought a 42" Hitachi plasma last February and unfortunately did not have his set to stretch, and with his young kids and wife watching more TV than him and watching mostly SD channels, his set now does have burn in, and you can see said vertical bars superimposed on the screen, and it's pretty darn noticeable. it sucks cuz it's a really nice set but he didn't heed the warnings and now he's really bummed about it. we tried doing the built-in "screen wipe" utility (or whatever it's called) multiple times but no luck.
i guess my point is plasma burn-in is not completely out of the question, even with the newer sets, so still use caution. but for clarity and a true HD, imo plasma really rocks. It's literally a year later and I still find myself saying "wow, would you look at that picture?"
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