gray squares; what are they?

I get a diary like the one on Obama, and in various places there are gray squares.

Is this a form of censorship?

What are they?

I'm beginning to think that there's censorship here. I get gray squares over diaries in various places, often covering information that I find relevant. The big diary with photos of Obama's visit to Iraq has a bunch.

What are they? are they censorship of some kind? are Trusted Users deciding that I'm unworthy of looking at certain things?

Is dk censoring diaries, or is it something else?

I would really like somebody to tell me what they are... and if you get them too.