Probably never. Unless they can make a dual-gpu card of a GTX 470 or better core, they won't do it. No sense in releasing one unless it is faster than a 5970.
Well there are a few variables to consider.
1) Fermi scales better with SLI than crossfire.
2) Fermi scales slightly better with core/shader mhz increases than 5800's (most likely due to fermi having lower fillrates than 5800's)
3) GF104's fully unlocked chip is 384 shader parts. If a 384 shader, 700mhz clocked part can outperform a 5850 and keep draw power low enough to make a dual card out of, SLI's better scaling may help it match/surpass an hd5970.
4) Time is against Nvidia in releasing a dual GPU part with their current chips. ATI is soon to release the 6x00 cards --- but then again, who knows if the TDP of the 6x00 cards will be low enough to make a dual card; and I highly, highly doubt ATI can top 5970 performance with their next gen cards being on the same node process.
I'm guessing they'll release a a gtx490, or 495 or whatever with two full unlocked GF104 cores. But I think Nvidia wants to clear out GF100 stock first, because 2 GF104 unlocked cores could be cheaper to make and sell than a gtx480 and will perform better than it. On the same note, I'm also guessing Fermi II will be launched this fall ---sooner than later, akin to what Nvidia did with the GeforceFX series (5800 --> 5900).