Really depends on what you need NIC, SAS controller and PCI-E slot wise. Going by what shows up on ebay, most are Supermicro, Intel and some Asus. My personal preference is Supermicro -> Intel -> Gigabyte/Asus/Asrock.Seconded.
There seems to be quiet a limited number of dual-socket boards around thankfully, but finding usable reviews seem harder. I'd love to see what kind of ATX/EATX boards people use with these proc's