I had a similar experience although my lines were vertical, the Samsung guys took it away to repair but alas they informed me the screen had gone
Upshot was to replace the screen just isnt cost effective as its cheaper to buy a new Plasma, so you could call the Samsung enginner out to confirm its gone or repairable.
If you search the forums theres a thread here about your rights etc etc and these things should last longer etc etc and you may well have recourse through your supplier. Its gotta be worth a shot , let us know how you get on i know of at least 2 people who have got new TV's after they broke down after a similar period to yours.