According to an article on theinquirer.net Nvidia SLI can work with more than just two cards. In fact, it's entirely possible to make four cards work with each other. The possibilities are indeed interesting - most notably in the area of using 2 dual-chip cards together.
Tier one companies such as Gigabyte and Asus already have cards with two 6600GT chips on one PCB and at CeBIT we noticed that Asus is working on an extremely big printed circuit board (PCB) with two 6800 chips.
What might happen in the future is that an Asus board with 6800 Ultra could become the record breaker as with proper drivers it might win the performance crown. Two cards with two chips on each card might perform quite well and I don't think you will have enough PCIe ports to plug more than two cards into anytime soon. Still, you never know what might happen as some of the motherboard manufacturers might decide to go for the board with more than two graphic PCIe ports.