From what I've read, some people swear by it, others don't. Personally, I ran the break-in slides for a good 30-40 hours, and then just let the TV run on DirecTV's HD movie channels when I wasn't hanging around the room. If I played video games for a couple hours during the first 100, I'd enable the TV's scrolling mode for a couple hours, or again, set the channel to DirecTV's HD fullscreen movies.
While I may not have done it as religiously as others, I did it because I know what type of care having a plasma TV involves. My father, however, has owned a couple plasma TV's since they came out, and he never did any sort of break-in. His original plasma simply crapped out within 2 years (it was a Sharp 42" with the ambient lighting feature. Amazing TV for its time.), and its replacement was a 50" LG which has withstood 6 years of countless hours of NFL, NHL, NASCAR, video games, etc., and has never had any problems whatsoever. The picture on it is becoming a little dated, and it does suffer a little more IR than my newer Samsung set, but absolutely no issues with burn-in, dead pixels, etc. to this day.
Compare that to my 10 month old Samsung (PN51D550), which has had a screen replacement due to an extreme amount of defective pixels (30+), and has just recently been submitted for another repair due to random power cycles, vertical banding, and spots of uneven screen brightness. Not to mention the fact that there has been some slight burn-in with even the utmost care. I'm beginning to wonder if people are taking precautions such as these due to the fact that TV's may not be built to their best quality nowadays.