Well, NVidia and ATI really don't have much competition* so I could see the DOJ looking into it. But, I think DOJ dropped it because they didn't have a case. ATI & Nvidia do have a natural duopoly right now simply because it's so expensive to get into the serious GPU-making business, If anything it seems like ATI and NVidia are in a massive price war, cranking out new modelss and dropping the old ones so fast to sub-$100 prices so fast.
*Competition: As far as I know, there's Intel's onboard, Via's onboard (which is more or less S3 Unichrome + extra MPEG-2 or H.264 decoding.. so not usable for 3D but nice for video playback). Matrox makes video cards for like medical, financial, and video wall use.. they basically specialize in multihead, the 4-head card with 3x splitters gets you a 12-head setup at 1280x1024 per head. What else is there for desktops right now? I haven't heard from XGI in a while (they were spun off of SiS's graphics division).