This month, British citizen Ross Anderson couldn't attend an awards ceremony in DC because of visa issues. (You can listen to his recorded acceptance speech.) I've heard of at least one other prominent cryptographer who is in the same boat. Is there some cryptographer blacklist? Is something else going on? A lot of us would like to know.
A weird paper was posted on the Cryptology ePrint Archive (working link is via the Wayback Machine), claiming an attack against the NSA-designed cipher SIMON. You can read some commentary about it here. Basically, the authors claimed an attack so devastating that they would only publish a zero-knowledge proof of their attack. Which they didn't. Nor did they publish anything else of interest, near as I can tell.
The paper has since been deleted from the ePrint Archive, which feels like the correct decision on someone's part.