Hezballah is smart enough to operate slightly under the radar screen. They don't hit Americans outside of Lebanon so people think we can avoid them here.
My point in mentioning Hezbollah was to demonstrate the fact Iran openly funds and supports known terrorist groups. I don't think it's out of the realm of possibility for the same regime to provide aid and comfort to an individual/group like Bin Laden and al Qaeda. It certainly wouldn't be difficult, from a logistics standpoint. Afghanistan, Pakistan, and Iran all border on each other.