I am studying in Canada. My citizenship is Iranian; I have to get a visa and a study permit to enter and study in Canada, like many other international students. I'll need a visa to return to Canada everytime I leave say for a conference or for a visit back home in Iran but I can usually get that visa. It will normally take no more than a month [I once had to wait for three months, but then they had lost my medical file].
If you are an international student studying in the United States, the procedure is very similar to the above, except that if you are from one of those trouble-making countries, including Iran.
Before Sept. 11th, the normal process for citizens of these "rogue" [a pre "axis of evil" term] countries included a one month background check and an a tough interview in which normally the visa applicants would have to prove that they had no intention of staying in the US beyond their intended study period. Iranian students who were planning to study in the US would thus be prepared for these interviews and would [with many difficulties still] have had a decent hope of getting their visas in time.
After Sept. 11th the procedure changed: The background check period increased dramatically and indefinitely. Many studenets were stranded and missed their research/course/teaching responsibilities and uncounted months of their time was wasted. The treatment of these students became that of common criminals and petty thieves instead of academics. In effect the new visa procedures have reduced the number of incoming graduate students (form all countries, actually) and reserach staff in American universities, up to a point that even some tutorial classes have been cancelled [Look at here and here, you can find more on their website on this issue]
I will not add my personal feelings and thoughts on this issue which has affected many of my friends but instead I invite everyone to have a look at this petition and add their words on this to be shared [Thanks to Pedram Mo'alemian][I just did some editing on this post].