Could have been something in your purse pressing up against it??
Still pixels can die for no specific reason, but usually it's only a few. Odd that yours would have a whole line, but based on the pic, that's what it looks like. It might be a cracked screen, but I can't tell.
The black line has gotten worse. In the picture it looks line one black line on top and another half a black line on the bottom. Well the "bottom" line has gotten longer and is now just as long as the top one... making one thick black line. It's a hard screen, not one of those soft ones that you can press against.
I ran my fingernail across the screen and I can't feel any cracks or breaks.
Does one dead pixel cause the others around it to die? I just think it's so weird that this line is getting progressively longer and thicker..
If you didn't get a new phone, then they should be able to give you one at the subsidized price even if you've already renewed. Also, Samsung will be releasing a new Galaxy S in the next month or so. Think you can wait?
Looks like some internal wiring isn't offering power to your LCD pixels. That tells me that you've probably got some wear and tear on your wiring - a common issue with flip phones after prolonged use and other types of phones where the components are somewhat "exposed."
One way to see if your phone can repair the issue is to remove the battery completely from the phone/outlet for a full 10 minutes to allow for discharge. Then power on with the battery again. This should power up some of the other pixels - otherwise you'll know they've been shorted.