Do the black marks go from the dock up to the top of the screen? I've heard of the backlighting in the monitors having problems that can cause that. If you could post a link to that thread on Apple's forums, that would be great.
And yes, the NVidia 8600M GT graphics cards are prone to failing. Mine overheated last week. Unfortunately, there is currently no fix from Apple or NVidia. Your best bet would be to get the AppleCare Protection Plan, if you don't already have it.
I believe that Apple has a 14 day return policy with no questions asked. If you're that worried about it, just take it back and wait until the problems have been fixed and don't buy a computer with an NVidia Geforce series graphics card, just to be safe.