SLI certified memory has additional data in the SPD(onboard data chip) which sets the timings to the optimum for a board that can read it.
To be honest, In my opinion it's a bit of a gimmick.
Just an easy way for someone to automatically set the memory to the specified parameters.
eg. I have some Geil 4-4-4-12 DDR2 6400 but when I install it the default timings are set to 5-5-5-18 !! to ensure it boots.
SLI memory, when enabled would automatically set to 4-4-4-12.
but is nothing special.
AD