My favorite blocked-letters hidden-message was during the first time the US killed Iraqis. The first President Bush went to the Raytheon death factory to praise those who made the Patriot war missiles. So the front page of the NYT featured a picture of the President with a giant missile looming in the background, with the word RIOT visible on it [Bush presumably blocking the first 3 letters].
I thought this was just genius. And despite some research/googling, I've never been able to find a copy of that photo. Of course, in China, now I have both google and the NYT blocked. But I'd love to have a copy of that.