You need to check first to see if it's the monitor or GPU.
Attach an external monitor to your iMac. Is the green line there? If it is, then you can assume it's a GPU problem. If not, it's your iMac display.
Regards.
I don't really have an external monitor to test this out on. Is there a way to link it to my Macbook and use the latter as an external monitor instead?
I have the same green line. Luckily, it's off to the right, but is still annoying. Is there any way I can deal with this myself?
iMac
OSX 7 Lion
i have a green line on my monitor that appeared all of a sudden. and its not going away.
MacbookPro 15" os x
i bought it 5 months ago