I am also having this same issue.
Per DennisWingo's instructions, I examined the left-hand cable and discovered that the brown wire in my inverter cable is (indeed) pinched and frayed through, and I confirmed continuity at the break with a multimeter.
Once I soldered and taped that wire, it made no difference. With only the inverter plugged in, I now have a nice bright backlight and grey screen with no lines.
Now, with the inverter assembly disconnected and the video cable connected, I can faintly see my vertical and horizontal lines if I use a bright light. From this, I am certain my inverter board and cable have nothing to do with the lines in the screen.
I downloaded a datasheet on the panel and tested, to the best of my ability, whether there were any broken or faulty LCD edge contacts that I could repair. I then slit and peeled back all the sheathing from the data cable to see if there were any pinched wires in THAT cable..
Nope, nothing.
Bottom line: I replaced the LCD panel, retaining my original inverter and data cable, with another 30-pin LG panel of the same model number (although not from an Apple computer). I don't want to be responsible for suggesting a vendor or a source though, YMMV.
Voila, no lines in the new panel! It has a slightly glossier finish than the Apple equivalent so it looks a little different to me.
So definitely the LCD panel itself, although as I mentioned, one of my inverter wires were also frayed as DennisWingo suggested. Perhaps there are actually more people with *more than one defect* in their LCD screen, which -- if undiagnosed -- might explain the various reports that replacing one single component, or replacing one single cable, do not fix the issue.
Sorry, no easy fix. But thats how I fixed mine. :-/