If we're to believe information posted on HKEPC, AMD is working on CrossFire for three cards and the RD790 chipset will be the first to support three cards. Although not a lot of information is known at this time, the addition of HyperTransport 3.0 and PCI Express 2.0 makes this chipset a clear candidate for new features.
Add to this 41 PCI Express lanes and its clear that AMD is working on something, although Physics is of course one of the most touted applications for a third card. However, HKEPC claims that the performance improvement of adding a third card raises CrossFire performance from 1.8x to 2.6x by going from two to three cards over a single card.
If AMD indeed can pull this off, then AMD should have a solid lead over Nvidia's SLI technology, as long as they can convince their customers to spend the extra money on a third card.
It's also worth mentioning the power characteristics of the RD790 chipset, as HKEPC is reporting that it will draw a mere 3W at idle and 10W at full load. Compare this to Intel's upcoming X38 chipse which draws 14.4W at idle and up to 35W at full load.
We'll have to wait until September before AMD has these chips in mass production. Hopefully we'll see some early boards before then and it will be interesting to see what AMD's partners can do with this chipset. Sadly, it will continue to use the SB600 southbridge, as the SB700 won't be ready until later this year.