ATI and AMD can do triple and even Quad crossfire on its RD790 boards. This chipset will be launched this year, probably in Q4 2007 and ATI is working to solve the last pieces of driver overhead problem to make triple and quad Crossfire possible.
ATI has found a way to render more than three frames in advance and it is investigating how much performance you can get from a three or four graphics card setup. If all goes well, some graphics records will be broken and this might force Nvidia to reconsider canning its Quad SLI project from a while ago.
Another challenge is to make all this work under Vista, the OS that has been proven as a really hostile one to graphics manufacturers and its driver teams.