It seems like three-way SLI might be closer than we expected with several websites having information about it today. There are some limitations as to who can actually run it, starting with the fact that you need to have a Nforce 680i or 680i LT based motherboard, at least until the 780i comes out. We're not sure about AMD support, as so far no-one has mentioned it, but it should be possible with the 680a SLI chipset.
The next requirement is a graphics card with two SLI connectors on the top, such as the 8800 GTX or Ultra. This limits the audience of three-way SLI even further. A new SLI connector is also needed and we're not sure where you'll be able to get one of these from, unless you buy a new motherboard, but Nvidia will surely figure out a way.
The three cards are then connected to each other by the primary card having links to the second and third card, while the second and third cards are connected to each other as well. It might sound confusing, but the new SLI connector takes care of all the brain work for you. For those that can afford this kind of a setup it should offer some additional performance, but we suggest you get a fast CPU and a high-res display, as otherwise it'll be wasted money. The suggested launch date is the 15th of November.
On a side not, Expreview has also got some benchmarks of an 8800 GT vs. an 8800 Ultra and you can check that out here
You can also find out a bit more about three-way SLI here