It is a brand new Crucible steel that Chris Reeve helped develop. It is supposed to be an improved version of S30V, mostly due to the addition of Niobium, a little bit like S110V is an improvement over S90V (and S110V also contains Niobium).
Apparently it has moved beyond the experimental phase, Sal was mentioning in the Spyderco Forum recently that they are planning a Mule with that steel.