I had the same problem with a 2008 17" unibody MBP. I was NOT about to give it to a Genius for a $500 fix. Not in my nature.
I knew if I pushed on the indentation just south of the space bar or below the Option / Control / Command keys it went away. That told me something was loose or coming disconnected. Logically thinking, right? Wrong.
This makes no sense, but I took the bottom off with an electronics screwdriver and touched around a bit. There is a component that is a black square just to the left of the right side fan when looking at the bottom. It's about the size of an American Quarter and has a screw at the top left and bottom right. Not much identification as far as letters or numbers. When I touched that, the lines went away. I screwed it tighter and they didn't go away after a reboot. But GET THIS... I LOOSENED them about 1/16th of a turn -- Just those two little Phillips Head screws.
Fixed. I have no idea what that component is, but it's been working perfectly for a few hours now with the bottom replaced so I don't think it was a heat problem / fan problem / graphics card problem / logic board problem.
Worth a try, I would suggest. :)