nVidia has created a SLI-Ready certification for various systems components, listing manufacturers as being ready when they have verified their equipment works reliably in a dual nVidia video card solution. Corsair is the first in the memory category to receive this certification, and have announced such on their site. What the test involves isn't completely clear, but the PDF available on Corsair's sight gives a general rundown.
As a part of the SLI certification process, each component must pass stringent series of tests including diagnostics, benchmarking, burn in, and card interoperability.
In particular, the certified memory is Corsair's XMS-5400C4 family, a line of DDR2 modules, currently only for LGA755 boards but soon for the new Athlon M2 socket as well. Of course, like all HCL's, it isn't a vendor-lock, and you are free to use whatever memory you want in your personal system.