nVidia is careful not to mention "SLI" anywhere in their drivers for the GX2. They call it "Multi GPU mode"
I guess the term "SLI" is reserved for multi-SLOT configurations only ;P (OK to be honest, I have never used SLI so I don't know if they ever make mention of "SLI mode" in the drivers)
Even if you run GPU-Z with the GX2 it says SLI is disabled, even though it is clearly enabled (says so in drivers and crysis reports multi GPU mode as well)
Not sure if that's because GPU-Z needs to come out with an update or what.