I've had cal/mag deficiencies (they tend to go hand in hand) before and that doesn't look like it. What it is, though, I'm not sure. But I've seen a lot of this stuff going around on this, and a couple of other forums, so it'd be nice to nail down a diagnosis.

On another post with spotting like this, they seemed to think it was a fungus of some sort. But they didn't sound too sure of themselves :P.