As this DailyKos piece makes clear, Ambassador Khaliizad, and by extension it's likely the US Government, thinks that Iran is VERY heavily, deeply involved. Problem with that theory is Occam's Razor. Essentially, this theory espouses parsimony in making theories, i.e. cut the theory down to its essentials. Don't add a lot of unneccessary detail that just clutters up the picture.
Is it necessary to conlcude that Iran is motivating the Iraqi insurgents to fight? Hardly. The fact that Iraq was invaded by a bunch of white, infidel Americans who appear to be more interested in Iraq's oil than in liberating Iraqis combined with the fact that Iraq's physical reconstruction has still not managed to raise Iraqi living standards even up to Iraq's February 2003 level and you have a completely sufficien explanation.
Do we need Iran to explain how Iraqi insurgents get their weapons? Again, hardly. Iraq was a very heavily armed society long before March 2003. Virtually every household had at least one AK-47. Also, the Iraqi government had ammunition dumps scattered all over the countryside. Those ammo dumps were not properly secured because the US Army was in a rush to get to Baghdad. The other major weapon of the insurgents is the IED, the Improvised Explosive Device. As the ammo dumps were not secured, this is a fully sufficien explanation for the ability of the insurgents to fight on.
We can't rule out Iranian involvement in Iraq, but it's hardly necessary to make the presumption that Iran must be involved.
Is Iran involved with Hezbollah? Again, there's no reason to think so. The Katyusha rockets (The Soviet Union originally produced them for use in World War II, so like the AK-47, it's difficult to trace where they ultimately came from.) that are landing all over Northern Israel could have been put into place many years ago as it would have been a pretty sure bet when Israel first pulled out of Lebanon in 2000 that Israel might someday return. Again, that doesn't rule out Iranian involvement, but it's hardly necessary to bring them into the picture to explain anything.
I strongly suspet that Iran is far less involved than the US thinks it is, but that Iran makes a good "Villain-of-the-moment" for the US to blame things on.
UPDATE: AIPAC, the lobbying group for Israel concocts a veritable Legion of Doom to describe all of the terrorist groups allegedly working out of Lebanon alongside Hezbollah. Deserves the conservative blogger's comment: "Interesting, if true."