I've actually always wondered this, so I might as well ask you.

When I first came across them I was sure it was a joke or a fraud, but people seem serious about seeing some kind of art in them.

So what is it? As best as I can tell it seems to be a form of http://en.wikipedia.org/wiki/Pareidolia

Yes, you could perhaps call it Pareidolia. When I look at the aforelinked image http://www.terraingallery.org/Pollock-Number-One-1948.jpg , I see many things, or rather the hints of things. I have a hard time believing it is not intentional, or that the paint was thrown on canvas at random.

While the form is chaotic, the palette and composition are delectably on target. I am very much reminded of this image: http://cache.foxsaver.com/thumbnails/2008/01/04/205959019l.j...

I don't see any characteristic shapes or figures in them; in fact, I'd say that would detract from it. If it's a sensory experience, a feeling transmitted in a very raw form.

It's something like seeing atonal free jazz.

It is also worth noting that, at least for me, seeing a tiny jpg of a Pollock can in no way compare to actually seeing the real thing. I was totally indifferent to his work right up until the point I stood face to face with an actual painting. The size and 3-dimensional textures and all the other little details that, to me, really make the works great, cannot be translated via an image.