Did NVIDIA make a G80 "X2" card? No. The G80 core is too big and too hot to stuff two of them onto a 'single' card. With G92's reduced power size and power consumption, it became much much more feasible to put two cores so close together. But even with G92's higher efficiency, the 9800GX2 is just adequately cooled.
If two GT200 cores (rumored TDP >200W each) were to be put in the same place, a dual-slot aircooler wouldn't be able to effectively dissipate that kind of heat. A triple-slot design would be cumbersome and perhaps too heavy, and there aren't enough enthusiasts with watercooling to justify having a 'watercooling only' card.
If/when GT200 is shrunk to 55nm, it might be possible, but not likely before then.
I also really doubt a monster like GT200 has "awesome margins." Yields won't be great with such a big core, and in a time where $200 cards can run most games at high settings on 1920x1200 monitors, it's going to be a hard sell. It's not like average gamers are really clamoring for more performance right now.