Are the attacks against US troops in Iraq acts of terrorism or guerrilla warfare? To answer this, here is something to think about.
If you read the works of Mao, Che, and others on this topic, they would call these attacks a valid form of geurrilla warfare. Why? To borrow from Martin Van Creveld: war is only considered war when each side is at risk of destruction at the hands of the other. Any form of lethal violence that doesn't risk mutual destruction is considered slaughter and not warfare. 9/11 was slaughter. The lives of the attackers were never really at risk except by own hands. It fits with our sense of what the word terrorism means. In Iraq, attacks against US military targets puts the attacker at risk.
The US government would call these attacks terrorism. Why? Modern states have had a legal monopoly on violence (but not one in actuality). In their eyes, any lethal violence that is not within the framework of a nation-state to nation-state struggle is terrorism. Modern states have also extended the definition of terrorism to include non-lethal attacks on property. In this way, "terrorism" is currently used as a word of approbrium by nation-states for all forms of unlawful violence. [John Robb's Weblog]
Guerrilla warfare, of course. Terrorism refers to attacks on innocent civilians, and the occupying forces are obviously a legitimate military target.
As Robb suggests, there is plenty of historical precedent for such guerrilla warfare, although since I'm not a socialist I'm more inclined to point out examples from American history such as Francis Marion and Robert Rogers.