larzke

Since my original post, the problem had magically disappeared and everything seemed as normal. Then some time later, the screen went completely dark, and (unlike before) hooking up to an external display did not give any screen either.

So it had to be the video card, thus requiring an entire motherboard replacement.

I ended up calling Dell Tech support and found out I had another 9 months of warranty left. Nice surprise. Guy came over the next day and replaced the motherboard, and everything worked as normal. He pulled a big ugly chunk of dust out of the fan -- the video chip had obviously overheated and died.

Lessons learned:
- blow out your fan with compressed air every month or so
- if you don't get video on your LCD but do get video on an external display, the video card can still be a problem (normally you'd blame the LCD)
- check if you have warranty before you buy a $120 LCD replacement on eBay

To feranick: I would indeed strongly suspect your NVIDIA chip, if only because they are known to be flawed in the D600 series and it sounds like you already tried a different LCD. I hope you have warranty because if you have to buy a replacement motherboard it may be more worthwhile to buy a new laptop.