I recently bought a 42" Panasonic Plasma. I haven't noticed any "burn-ins", but I'll look out for it. I liked the plasmas I saw better than the LCDs.
We've (my family) had a 52" Pioneer? plasma TV for over a year now (last October 13th) and have yet to see any burn-in... My friends have had plasmas for even longer and have had zero burn-in. What the hell do you guys do with your TVs to burn them in so much?
Generally speaking, they're only vulnerable during the first 100 hours of use. Obviously, there's no magic number for all displays, or even for one display, but the bottom line is that the problem goes away as the phosphors age. Also, as has already been discussed, the newish ones (yours included) burn the images out from normal use (changing channel once in a great while), so the screen won't be ruined even if you do manage to burn in an image.
[edit] also worth noting that even networks that have logos do not display them during commercials, and that is probably enough to keep a screen >100hrs old burn-free.