Lest you think this is a problem only for Africans & USA: I was denied visa, going to present my paper at NIPS 2010 in Vancouver, Canada. Czech citizen (EU), PhD student at the time, with obvious ties to CZ. I fully respect each country's right to refuse anyone, including without providing a reason (as was the case with me). I see it as "their loss" :-) You pesky computer scientists, stay away!
http://www.owl232.net/papers/immigration.htm
I now find this to be a very interesting and subtle ethical question