The AP's story on Iran's CNN ban says:

"The ban marks the second time in a year that Iran has acted against an international broadcaster because of its coverage. Last April, Iran suspended the nationwide operations of Arab TV network Al-Jazeera, accusing it of inflaming violent protests by the Arab minority in its southwest.

Al-Jazeera was believed to have been the first news outlet to broadcast news of the violence unrest in Khuzistan province, which erupted after rumors spread of an alleged government plan to move non-Arabs into the oil-rich city of Ahvaz.

Al-Jazeera has not yet been allowed to return to Iran."