During IDF the engineers at Intel said that the 'ideal' memory configuration for a high-end Yorkfield is 2x1GB DDR3-1600 in order to be benchmarked against the Nehalem with 3x1GB DDR3-1066. If you don't believe us then check out the slide they showed us.
So, when you see the benchmarks come out it is likely most people will follow this 'tip' form Intel, but have no fear Legit Reviews will continue to test platforms on Vista Ultimate 64-bit with 4GB of memory on our Yorkfield platforms as that it what gamers and enthusiats are running. With memory prices so cheap right now not too many people are running 2GB on X38/X48 platforms and DDR2 is still the popular choice on current generation PCs.
We've also heard that single sided modules perform best on the new integrated memory controller as well as overclock better. Maybe that is the real reason behind this suggestion. Good thing it is just that - a suggestion on how to benchmark.