Exactly which 'failed states' do the terrorists that are a problem for them emanate from? Saudi Arabia? Egypt? Michigan?
I'm just not convinced that stability is such a big goal for the U.S. occupation of Iraq that a guerrilla insurgency would fundamentally harm the main mission. While chaos in Iraq is certainly bad for military morale, it may be just fine in other ways. If they want to control the country and the oil under it, it's easy when it's fractured and there's no state or government, but that's not the same as wanting to sell the oil, which might indeed require a smoothly-running protectorate. Right now I'd argue they want to control it, not sell it.
While a guerrilla resistance movement in Iraq can raise the political price within the U.S. (let's make sure it does) it's probably not an obstacle to the main goal there, which is the continued incapacitation and vivisection of a regional power (and, I'm with Yoshie on this one, the defeat of the Al Aqsa Intifada.)
Jenny Brown