http://www.supermicro.com.tr/press111708.cfm.htm
X58 is also used for server boards. "Design changes, however, have been made to the IOH component of the Intel® 5500 & 5520 chipset and the Intel® X58 Express chipset and will require customers to perform re-qualification. Please see the Intel® 5500 & 5520 chipset OR Intel® X58 Express chipset Specification Update"
Because of this, I took a chance and bought a Kingston 12Gb kit off ebay.
http://www.newegg.com/Product/Product.aspx?Item=N82E16820139720
NewEgg priced @ $179, sealed package, for $37 knowing it is Apple ECC memory. I dropped it in my X58 Asus SaberTooth motherboard w/X650 CPU. Granted the ram is 1066, so wasn't expecting to overclock it much, but never changed bios settings from Patriot Sector 7 DDR3 1600 ram. To my surprise, it came right up running 1576Mgz. and passed all benchmarks same as the previous ram. ECC ram (unregistered) does work. New 4Gb sticks @ $12.33 ea. I'm smiling
Kingston Technology Lifetime warranty.
Known Compatibility list for KTA-MP1066K3/12G
Apple Mac Pro DDR3 (Early 2009)
Apple Xserve Xeon DDR3 (April 2009)
Apple - Mac Pro DDR3 (Mid 2010) 1CPU
Apple - Mac Pro DDR3 (Mid 2010) 2CPU
Apple - Mac Pro DDR3 (Mid 2012) Quad-Core, 6-Core, 12-Core