Various sources have confirmed the existence of a dual G80 card. I wrote about this earlier, here.
The card is real, and a few people we talked about call it Geforce 8850 GX2. It is of course a dual card with two G80 chips on the board.
From what we've learned it should be clocked at higher than 600 MHz core clock, and again an odd memory number of 896 MB.
One source claims that the card will have 128 Shaders, but from what we learned before you should expect 96x2 or totally 192 Shader units.
It will certainly be faster than any overclocked G80 or 8800 GTX today and it should almost coincide with the R600 launch, coming before the end of May.