I'm pretty sure it has nothing to do with Black English Vernacular, especially "what it is?" which is just another way of saying what's happening, what's going on, what's up? And I think, Roger, you are tilting at windmills because the phrase is actually very useful when people are trying to read too much into a situation or trying to put some spin or nuance that they have created. So chill out on this particular saying because you're reading too much into it--it is what it is!

I also like Tony's definition (10 March 2010).