Lines are horizontal.
They are in the centre, they do not go all the way across screen.
they look more like boundaries' than lines!
my TV is 18 months old.
I can see lines more when playing DVDs, that are not HD. if the picture is busy & in HD, then it does not show up as much.
Using the menu, the lines are there, but are very hard to see.
Its an LCD display panel problem but I doubt if samsung will help you here because it will be considered to be within tolerance.
As the set is 18mths old you could take this up with samsung however to see what their take is on it you could contact them via the website www.samsung.com.
LCD panel failure in general is very common I see it in many guises on a daily basis this however wont be considered failure particularly if the lines are not seen at 8 feet or more from the set
Let me know if you need more info or further clarification
I am sorry about this and its not what you want to hear however please remember to rate the answer as 3 stars or better