Single-chip Fermi and the card codenamed GF100 should launch in March. This should be the launch time frame and a dual-chip version might follow in roughly one+ months after the official single-chip launches.

This is roughly the plan as it will take some time to prepare this dual headed beast, especially since Fermi will be the hottest chip in Nvidia's history. Naturally,  putting two such chips on the same card won't be a walk in the park.

The important thing is that dual-chip card is possible and Nvidia is working on it. The big issue is of course that it will take roughly three months from today before you will be able to buy one, whereas ATI's dual-GPU card has been selling for more than a month, closer to two now.