From last year's visit:
The State Department had to seek a special waiver for Ahmadinejad from the Department of Homeland Security because of unresolved allegations that he was involved in the 1979-81 takeover of the U.S. Embassy in Tehran, in which 52 hostages were held for 444 days. - Washington Post
He is scheduled to speak this next Tuesday at the UN, provided he gets a visa. According to this news report (US Presidential Nominee Calls for Talks to Iran - FARS News) he is still waiting for one.
Now eleven Republican Senators have asked the State Department not to issue Ahmadinejad a visa:
Dear Secretary Rice:The 63rd session of the United Nations General Assembly will begin on September 16, 2008 at the United Nations Headquarters in New York. We are aware that President of Iran, Mahmoud Ahmadinejad, and his officials want to participate as members of the United Nations (UN).As you know, the Reagan Administration denied Palestine Liberation Organization leader, Yasser Arafat, a visa to participate at the UN General Assembly in 1988 for his connections with terrorist acivities. We oppose President Ahmadinejad’s participation at the UN and ask for you to exercise your authority to deny his entry to the United States.President Ahmadinejad is a clear adversary of the United States and other democratic nations that uphold liberty and freedom. Iran’s leaders continue to pursue the development and acquisition of nuclear weapon capabilities. They continue to support terrorist organizations around the world, which have been linked to training insurgents in Iraq. Iran also regularly threatens our close ally, Israel. Most recently, President Ahmadinejad called Israel a “germ of corruption” and stated that they will be “removed soon”. These actions demonstrate Iran’s destructive intentions rather than a constructive dialogue.We understand that President Ahmadinejad’s presence in the United States is permitted under umbrella of P.L. 80-357, the United Nations Headquarters Agreement. However, we must convey our belief that at this time, Iranian leaders should not be allowed to enter the United States, even under the auspices of the U.N. It is unfortunate that the United Nations allows Iran to participate in the General Assembly. When leaders are openly working against the international community‘s efforts to address problems of concern, we should not provide them a bully pulpit.Thank you for your attention to this matter and we look forward to your response.Sincerely, [Note: Republican Senators] John Barrasso Wayne Allard, Jim Bunning, Christopher Bond, Richard Burr, Saxby Chambliss, Thad Coburn, Jim Inhofe, Lindsey Graham, John Thune, George Voinovich. Mike Enzi - (Found at Little Chicago Review)
* Note that Senator Clinton was ready to speak out and take a stand against him.