I run a pair of GTX 470s in SLI on the ASRock Extreme6 P67. They scale well (alomost a 90% performance boost), but the big issues are power and heat. Unless you are running a fully water cooled rig, you will have to be very mindful that the GTX Fermi GPUs are HOT, HOT HOT! Your case cooing has to be well managed, as well as the GPU cooling. I modded out a PCNY GTX 470 with the Zalman 2000F cooler and placed it in the top slot (as the top GPU runs hottest). You will need to use some aftermarket cooling or get non-reference cards (if you don't water cool).
Your PSU will have to be 1200 W + if you want to have sufficient overhead to power these power-hogs.
BTW the info on scaling 3 GTX 480s in SLI is similar to the GTX 570s and for the most part it is not worth the coin: http://benchmarkextreme.com/Articles/GTX%20570/P21.html