Sadly it means a new screen or board.
Apple did have issues with a lot of screens though, and for certain models will still replace them at no cost, but I believe it may only apply to the early Intel iMacs. A friend of mine had one done recently.
I would have a google around to see if it's a very common issue on your particular model, and see if anyone else has been lucky enough to have it changed.
If not, it's going to be a case of getting used to the lines until they really take over, or spending some $$ I'm afraid.