If we did the sums right that means that Nvidia decided to followed up with SLI almost immediately after acquiring 3Dfx back in December 2000. Well, it did take some months off this Voodoo legacy but returned surprisingly fast to the idea.It would not be that unlikely if ATI also decides to launch their own version of SLI, they could always ask their friends at Alienware to help them with their Video Array technology. But drivers will still be the hard part..
This means that there are lot of software engineering hours invested into this development and that such a complex thing simply takes much time and planning.
Nvidia is still proud of its SLI stuff and we will all see some numbers after Nforce 4 SLI introduction that is scheduled after the Nforce 4 Ultra/plain introduction.
Let's wait and see how those drivers will turn out. Oh yes I was told that 65 drivers with support for SLI cards will get an WHQL certificate. If you are using two cards and SLI mode it's twice as hard to get that stamp of approval but Nvidia is sure that it can get it.
More info about the development of NVIDIA's SLI here