"....the NSA people leaped into action, and immediately sent me a short email written with a lot of tact:
'“The trouble you are having is regrettableSorry you won’t be able to come to our conference. We have submitted our program and did not include you on it.'"
He goes on to suggest this is more cock-up than conspiracy, and is simply a consequence of the US visa bureaucracy "collapsing under its own weight".
The full email appears here.