If I have to bet, I would say that ATI will have theirs out first, since their desktop counterparts are already on the market, although availability is still not good. So I would expect ATI to be releasing these as early as early Dec, to catch the last part of the holiday shopping season, or as late as March.
It also depends on how much resources they will need to devote to R580 development, which will also due out sometime Q1-Q2 next year; they are already under the gun, so to speak, for getting into two big console video deals.
NV on the other hand, seems to be content to let the 6800 series to handle the mid-range stuff, with the 6600GT shoring up the lower part of the mid-end. The 7600Go will eventually come out, but I wouldn't be waiting for it if I was planning on buying a new NB.