It's a very worrisome situation with regard to 790i, in my personal opinion. Usually a small revisions are denoted as A0 -> A1 -> A2, etc.. But 790i has apparently been under a couple major revisions and I heard about C0 and C1 stepping for the 790i SPP.

We were also able to get our hands on a C0 engineering sample 790i board for testing. Let’s just say that the experience was … character building. Running a QX9770 and 1333Mhz DDR3 at 9:9:9:24, we had what could best be described as a very rough time getting 3-way and even quad SLI with two 9800 GX2 boards to work in this system. We were lucky to get the numbers we did get. Let’s take a look at what we tested with
http://www.anandtech.com/video/showdoc.aspx?i=3275&p=2