Dieter is one of many stories of people changing travel plans and missing conferences because of America's tougher requirements and slower processing of foreign immigration applications. An Indian graduate student with a paper at next week's Complexity conference could not get a visa in time. I would not be surprised if many graduate students will not start the fall semester on time awaiting my government's blessing to come to study here.
This is a story I have told before and will likely tell again. I understand the need for security but most scientific progress happens through collaboration and preventing or delaying this collaboration holds back the advancement of knowledge. Not since the 80's have we seen such a limitation on traveling though this time in reverse. During the cold war several countries would not let many of their best scientists out; these days we don't allow many of the world's best scientists in.