I own many samsung products, i have spent to the tune of 3000 USD on them & a 51" Plasma TV is one of them, within 4 months of owning the product; lines started appearing in the panel, like the one you have described here and in 15 months the panel is literally disintegrating line by line on a daily basis. I have even started a blog to describe their horrible quality practices right here :
Please visit the blog it has some pictures too.
I am also experiencing black line on my 3 year old Samsung. A couple phone calls and an hour and a half later, I am too frustrated. Not getting answers and customer service acting like if they don't know the issue. I have not even been able to get a price on how much it would cost to repair, although I guess between $ 500 to $ 800. The price of a new TV. Seems to me that Samsung does not get about loosing market shares.