Iran is the main source for funds, training, weapons, and other support for Hezbollah. There is at least one known link between Hezbollah and Al Qaeda (a meeting in 2002). The President of a country which openly supports terrorists should not be in this country for any reason.

After doing some searching, Iran has a slightly more solid link to Bin Laden than I thought:

http://www.meforum.org/article/670